Add mentions of theme settings and header/footer as supported catalog files in the contributor documentation

Author: d-ronnqvist

Sources/SwiftDocC/SwiftDocC.docc/SwiftDocC.md

@@ -11,6 +11,8 @@ DocC comes with built-in support for several types of input files. You organize
  - Tutorial files that include dynamic, learning content.
  - Asset files like images, videos, and archived projects for download.
  - An `Info.plist` file that contains metadata about the documentation catalog.
+ - A pair of `header.html` and `footer.html` files to use as a custom header and footer for the rendered output.
+ - A `theme-settings.json` with theming customizations for the rendered output.
 
 SwiftDocC provides the APIs you use to load a documentation catalog, parse the symbol-graph meta-information, extract symbol documentation, and optionally pair that symbol documentation with external file content. DocC represents the compiled documentation in an in-memory model that you can further convert in a persistable representation for writing to disk.
 

Sources/SwiftDocC/SwiftDocC.docc/SwiftDocC/CompilerPipeline.md

@@ -21,6 +21,8 @@ When a ``DocumentationWorkspaceDataProvider`` find as documentation catalog, it
 - Authored markup files with an `.md` extension.
 - Authored tutorial files with a `.tutorial` extension.
 - Additional documentation assets with known extensions like `.png`, `.jpg`, `.mov`, and `.zip`.
+- A pair of `header.html` and `footer.html` files to use as a custom header and footer for the rendered output.
+- A `theme-settings.json` with theming customizations for the rendered output.
 
 You can organize the files in any way, as long as the `Info.plist`, `header.html`, `footer.html`, and `theme-settings.json` files are the root of the catalog directory tree. 
 Here is an example of a documentation catalog that groups topic files in logical groups with an additional directory for shared asset files: