[docs] Add First Pull Request guide and Getting Started guide.

This improves upon the existing documentation to provide a clearer end-to-end
workflow for new contributors and people who wish to build the toolchain
locally but do not intend to submit patches.

We also provide more directions for systematically utilizing our existing
documentation.

Author: varungandhi-apple

Brewfile

@@ -1,2 +1,3 @@
 brew "cmake"
 brew "ninja"
+brew "sccache"

README.md

@@ -50,14 +50,9 @@ To learn more about the programming language, visit [swift.org](https://swift.or
 
 - [Contributing to Swift](#contributing-to-swift)
 - [Getting Started](#getting-started)
-  - [System Requirements](#system-requirements)
-  - [Getting Sources for Swift and Related Projects](#getting-sources-for-swift-and-related-projects)
-  - [Building Swift](#building-swift)
   - [Swift Toolchains](#swift-toolchains)
   - [Build Failures](#build-failures)
-- [Testing Swift](#testing-swift)
 - [Learning More](#learning-more)
-- [Build Dependencies](#build-dependencies)
 
 ## Contributing to Swift
 
@@ -77,206 +72,15 @@ well. For more, see the [Code of Conduct](https://swift.org/community/#code-of-c
 
 ## Getting Started
 
-These instructions give the most direct path to a working Swift development
-environment. To build from source you will need about 2 GB of disk space for the
-source code and up to 70 GB of disk space for the build artifacts with full
-debugging. Depending on your machine, a clean build can take a few minutes to
-several hours. Naturally, incremental builds are much faster.
+If you are interested in:
+- Contributing fixes and features to the compiler: See our
+  [How to Submit Your First Pull Request guide](/docs/HowToGuides/FirstPullRequest.md).
+- Building the compiler as a one-off: See our [Getting Started guide][].
+- Building a toolchain as a one-off: Follow the [Getting Started guide][]
+  up until the "Building the project" section. After that, follow the
+  instructions in the [Swift Toolchains](#swift-toolchains) section below.
 
-Once you are able to build things successfully and have a compile-test-debug
-loop going, check out the [development tips](docs/DevelopmentTips.md) for
-better productivity while working on the compiler.
-
-You can also skim [docs/README.md](/docs/README.md) to understand what
-high-level documentation is available.
-
-### System Requirements
-
-macOS, Ubuntu Linux LTS, and the latest Ubuntu Linux release are currently
-supported as host development operating systems.
-
-Please make sure you use Python 2.x. Python 3.x is not supported currently.
-
-#### macOS
-
-To build for macOS, you need [Xcode 12 beta 3](https://developer.apple.com/xcode/resources/).
-The required version of Xcode changes frequently, and is often a beta release.
-Check this document or the host information on <https://ci.swift.org> for the
-current required version.
-
-Swift's build tooling is meant to support spaces in the paths passed to them,
-but using spaces sometimes tickles bugs in Swift's build scripts or the tools
-they rely on. For example, [SR-13441](https://bugs.swift.org/browse/SR-13441)
-is caused by a space in the Xcode path used on macOS. If you see Swift's build
-tooling misbehave due to a space in a path, please
-[report the bug on the Swift bug tracker](https://swift.org/contributing/#reporting-bugs)
-and then change the path to work around it.
-
-You will also need [CMake](https://cmake.org) and [Ninja](https://ninja-build.org),
-which can be installed via a package manager:
-
-**[Homebrew](https://brew.sh/)**
-
-    brew install cmake ninja
-    
-You can also use [homebrew-bundle](https://github.com/Homebrew/homebrew-bundle)
-from the root of this repository's working directory to install all of these
-dependencies:
-
-    brew bundle
-
-**[MacPorts](https://macports.org)**
-
-    sudo port install cmake ninja
-
-Instructions for installing CMake and Ninja directly can be found [below](#build-dependencies).
-
-#### Linux
-
-For Ubuntu, you'll need the following development dependencies:
-
-```
-sudo apt-get install    \
-  clang                 \
-  cmake                 \
-  git                   \
-  icu-devtools          \
-  libcurl4-openssl-dev  \
-  libedit-dev           \
-  libicu-dev            \
-  libncurses5-dev       \
-  libpython-dev         \
-  libsqlite3-dev        \
-  libxml2-dev           \
-  ninja-build           \
-  pkg-config            \
-  python                \
-  python-six            \
-  rsync                 \
-  swig                  \
-  systemtap-sdt-dev     \
-  tzdata                \
-  uuid-dev
-```
-
-**Note:** LLDB currently requires at least `swig-1.3.40` but will successfully build
-with version 2 shipped with Ubuntu.
-
-**Note:** For Ubuntu 20.04, use `libpython2-dev` in place of the libpython-dev package above.
-
-### Getting Sources for Swift and Related Projects
-
-First create a directory for all of the Swift sources:
-
-    mkdir swift-source
-    cd swift-source
-
-**Note:** This is important since update-checkout (see below) checks out
-repositories next to the Swift source directory. This means that if one clones
-Swift and has other unrelated repositories, update-checkout may not clone those
-repositories and will update them instead. Be aware that `update-checkout`
-currently does not support paths with non-ASCII characters. If such characters
-are present in the path to `swift-source`, `update-checkout` will fail.
-
-**Via HTTPS**  For those checking out sources as read-only, HTTPS works best:
-
-    git clone https://github.com/apple/swift.git
-    ./swift/utils/update-checkout --clone
-
-**Via SSH**  For those who plan on regularly making direct commits,
-cloning over SSH may provide a better experience (which requires
-[uploading SSH keys to GitHub](https://help.github.com/articles/adding-a-new-ssh-key-to-your-github-account/)):
-
-    git clone git@github.com:apple/swift.git
-    ./swift/utils/update-checkout --clone-with-ssh
-
-### Building Swift
-
-The `build-script` is a high-level build automation script that supports basic
-options such as building a Swift-compatible LLDB, building the Swift Package
-Manager, building for various platforms, running tests after builds, and more.
-
-There are two primary build systems to use: Xcode and Ninja. The Xcode build
-system allows you to work in Xcode, but Ninja is a bit faster and supports
-more environments.
-
-First, make sure that you're in the swift directory:
-
-    cd swift
-
-To build using Ninja, run:
-
-    utils/build-script --release-debuginfo
-
-When developing Swift, it helps to build what you're working on in a debug
-configuration while building the rest of the project with optimizations. Below
-are some examples of using debug variants:
-
-    utils/build-script --release-debuginfo --debug-swift # Swift frontend built in debug
-    utils/build-script --release-debuginfo --debug-swift-stdlib # Standard library built in debug
-    utils/build-script --release-debuginfo --debug-swift --force-optimized-typechecker # Swift frontend sans type checker built in debug
-
-Limiting the amount of debug code in the compiler has a very large impact on
-Swift compile times, and in turn the test execution time. If you want to build
-the entire project in debug, you can run:
-
-    utils/build-script --debug
-
-For documentation of all available arguments, as well as additional usage
-information, see the inline help:
-
-    utils/build-script -h
-
-#### Xcode
-
-To build using Xcode, specify the `--xcode` argument on any of the above commands.
-Xcode can be used to edit the Swift source code, but it is not currently
-fully supported as a build environment for SDKs other than macOS. The generated
-Xcode project does not integrate with the test runner, but the tests can be run
-with the 'check-swift' target.
-
-#### Build Products
-
-All of the build products are placed in `swift-source/build/${TOOL}-${MODE}/${PRODUCT}-${PLATFORM}/`.
-If macOS Swift with Ninja in DebugAssert mode was built, all of the products
-would be in `swift-source/build/Ninja-DebugAssert/swift-macosx-x86_64/`. It
-helps to save this directory as an environment variable for future use.
-
-    export SWIFT_BUILD_DIR="~/swift-source/build/Ninja-DebugAssert/swift-macosx-x86_64"
-
-#### Ninja
-
-Once the first build has completed, Ninja can perform fast incremental builds of
-various products. These incremental builds are a big timesaver when developing
-and debugging.
-
-    cd ${SWIFT_BUILD_DIR}
-    ninja swift-frontend
-
-This will build the Swift compiler, but will not rebuild the standard library or
-any other target. Building the `swift-stdlib` target as an additional layer of
-testing from time to time is also a good idea. To build just the standard
-library, run:
-
-    ninja swift-stdlib
-
-It is always a good idea to do a full build after using `update-checkout`.
-
-#### Using Xcode
-
-To open the Swift project in Xcode, open `${SWIFT_BUILD_DIR}/Swift.xcodeproj`.
-It will auto-create a *lot* of schemes for all of the available targets. A
-common debug flow would involve:
-
- - Select the 'swift-frontend' scheme.
- - Pull up the scheme editor (⌘⇧<).
- - Select the 'Arguments' tab and click the '+'.
- - Add the command line options.
- - Close the scheme editor.
- - Build and run.
-
-Another option is to change the scheme to "Wait for executable to be launched",
-then run the build product in Terminal.
+[Getting Started guide]: /docs/HowToGuides/GettingStarted.md
 
 ### Swift Toolchains
 
@@ -335,26 +139,24 @@ compiler crashes.
 
 ### Build Failures
 
-Make sure you are using the [correct release](#macos) of Xcode.
+Try the suggestions in
+[Troubleshooting build issues](/docs/HowToGuides/GettingStarted.md#troubleshooting-build-issues).
+
+Make sure you are using the
+[correct release](/docs/HowToGuides/GettingStared.md#installing-dependencies)
+of Xcode.
 
 If you have changed Xcode versions but still encounter errors that appear to
 be related to the Xcode version, try passing `--clean` to `build-script`.
 
 When a new version of Xcode is released, you can update your build without
 recompiling the entire project by passing the `--reconfigure` option.
 
-Make sure all repositories are up to date with the `update-checkout` command
-described above.
-
-## Testing Swift
-
-See [docs/Testing.md](docs/Testing.md), in particular the section on [lit.py](docs/Testing.md#using-litpy).
-
 ## Learning More
 
-Be sure to look through the [docs](https://github.com/apple/swift/tree/master/docs)
-directory for more information about the compiler. In particular, the documents
-titled [Debugging the Swift Compiler](docs/DebuggingTheCompiler.md) and
+Be sure to look at the [documentation index](/docs/README.md) for a bird's eye
+view of the available documentation. In particular, the documents titled
+[Debugging the Swift Compiler](docs/DebuggingTheCompiler.md) and
 [Continuous Integration for Swift](docs/ContinuousIntegration.md) are very
 helpful to understand before submitting your first PR.
 
@@ -378,41 +180,3 @@ Another source of documentation is the standard library itself, located in
 `stdlib`. Much of the language is actually implemented in the library
 (including `Int`), and the standard library gives some examples of what can be
 expressed today.
-
-## Build Dependencies
-
-### CMake
-[CMake](https://cmake.org) is the core infrastructure used to configure builds of
-Swift and its companion projects; at least version 3.16.5 is required.
-
-On macOS, you can download the [CMake Binary Distribution](https://cmake.org/download),
-bundled as an application, copy it to `/Applications`, and add the embedded
-command line tools to your `PATH`:
-
-    export PATH=/Applications/CMake.app/Contents/bin:$PATH
-
-On Linux, if you have not already installed Swift's [development
-dependencies](#linux), you can download and install the CMake
-package separately using the following command:
-
-    sudo apt-get install cmake
-
-
-### Ninja
-[Ninja](https://ninja-build.org) is the current recommended build system
-for building Swift and is the default configuration generated by CMake. [Pre-built
-packages](https://github.com/ninja-build/ninja/wiki/Pre-built-Ninja-packages)
-are available for macOS and Linux distributions. You can also clone Ninja
-next to the other projects and it will be bootstrapped automatically:
-
-**Via HTTPS**
-
-    git clone https://github.com/ninja-build/ninja.git && cd ninja
-    git checkout release
-    cat README
-
-**Via SSH**
-
-    git clone git@github.com:ninja-build/ninja.git && cd ninja
-    git checkout release
-    cat README