Branches: divide branches into categories

In preparation for adding topic branches such as `rust-pci`.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

src/Branches.md

@@ -1,24 +1,26 @@
 # Branches
 
-Currently we maintain the following branches.
+## Main branches
 
-## `rust-next`
+Currently we maintain the following main branches.
+
+### `rust-next`
 
 [`rust-next`](https://github.com/Rust-for-Linux/linux/tree/rust-next) is the branch that contains new Rust features to be submitted during the next merge window of the Linux kernel.
 
 Changes to this branch land via patches sent to the mailing list.
 
 It is part of [`linux-next`](https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/).
 
-## `rust-fixes`
+### `rust-fixes`
 
 [`rust-fixes`](https://github.com/Rust-for-Linux/linux/tree/rust-fixes) is the branch that contains Rust fixes for the current cycle of the Linux kernel.
 
 Changes to this branch land via patches sent to the mailing list.
 
 It is part of [`linux-next`](https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/).
 
-## `rust-dev`
+### `rust-dev`
 
 [`rust-dev`](https://github.com/Rust-for-Linux/linux/tree/rust-dev) is an experimental branch for integration purposes. It is a queue for patches that "look good enough".
 
@@ -30,7 +32,11 @@ Its intended use cases are:
 
 Note that this branch may be updated/rebased frequently and it might be gone in the future.
 
-## `rust`
+## Past branches
+
+These branches are unmaintained, archived, deprecated, frozen or do not exist anymore in the repository.
+
+### `rust`
 
 [`rust`](https://github.com/Rust-for-Linux/linux/tree/rust) was the original branch where development happened for two years before Rust support was merged into the kernel.