Added information on how to create a Documentation Catalogue. (#1165)

* Added information on how to create a Documentation Catalogue.

* US spelling...

* Missing quote marks in image references.

* Update Sources/docc/DocCDocumentation.docc/documenting-a-swift-framework-or-package.md

Co-authored-by: David Rönnqvist <ronnqvist@apple.com>

* Undo changes to image references.

* Explanations of what a catalogue is in filesystem terms.

---------

Co-authored-by: David Rönnqvist <ronnqvist@apple.com>

Author: Diggory

Sources/docc/DocCDocumentation.docc/adding-supplemental-content-to-a-documentation-catalog.md

@@ -4,7 +4,7 @@ Include articles and extension files to extend your source documentation comment
 
 ## Overview
 
-A documentation catalog contains files that enrich your source documentation
+A documentation catalog (a directory with a '.docc' extension) contains files that enrich your source documentation
 comments. Two types of files that documentation catalogs can contain are
 articles and extension files.
 

Sources/docc/DocCDocumentation.docc/documenting-a-swift-framework-or-package.md

@@ -47,7 +47,7 @@ By default, DocC compiles only in-source symbol documentation and then
 groups those symbols together by their kind, such as protocols, classes, 
 enumerations, and so forth. When you want to provide additional content or 
 customize the organization of symbols, use a documentation 
-catalog.
+catalog (a directory with a '.docc' extension).
 
 DocC combines the public API information from the Swift compiler with the 
 contents of the documentation catalog to generate a much richer DocC Archive.
@@ -71,6 +71,8 @@ step-by-step, interactive content. For more information, see
 > Important: To use a documentation catalog in a Swift package, make sure the 
 manifest's Swift tools version is set to `5.5` or later. 
 
+To associate a documentation catalog with a target in your package, add the catalog to the same directory as the target's other source files (`[PackageRoot]/Sources/[TargetName]` by default).
+
 ## Building, Publishing, and Previewing Documentation with the DocC Plug-in
 
 The preferred way of building documentation for your Swift package is by using