Docs: Add @PageColor to the customization guide (#691)

Author: natikgadzhi

Sources/SwiftDocC/Semantics/Metadata/Metadata.swift

@@ -23,6 +23,7 @@ import Markdown
 /// - ``TechnologyRoot``
 /// - ``DisplayName``
 /// - ``PageImage``
+/// - ``PageColor``
 /// - ``CallToAction``
 /// - ``Availability``
 /// - ``PageKind``

Sources/docc/DocCDocumentation.docc/customizing-the-appearance-of-your-documentation-pages.md

@@ -79,7 +79,8 @@ of a green hue.
 
 As a general rule, the default color properties provided by DocC assumes a
 naming convention where "fill" colors are used for backgrounds and "figure"
-colors are used for foreground colors, like text.
+colors are used for foreground colors, like text. Note that colors defined in 
+`theme-settings.json` will be used across all pages of your documentation bundle.
 
 > Tip:
 > For a more complete example of a fully customized documentation website, you
@@ -239,6 +240,16 @@ be added as necessary for other features.
 }
 ```
 
+### Customizing the appearance of specific pages
+
+Aside from the customizations available via `theme-settings.json`, Swift DocC
+provides several <doc:Metadata> directives that allow you to customize just
+one specific Article page.
+
+Most notably:
+- ``PageImage`` allows you to set a header image of a page.
+- ``PageColor`` allows you to set an accent color of a page.
+
 [1]: https://github.com/apple/swift-docc/blob/main/Sources/SwiftDocC/SwiftDocC.docc/Resources/ThemeSettings.spec.json
 [2]: https://drafts.csswg.org/css-variables/
 [3]: https://mportiz08.github.io/swift-docc/documentation/docc