Add AI documentation skills

.ai/skills/mkdocs/references/upstream/mkdocs-1.6.1-docs/dev-guide/README.md

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+# Developer Guide
+
+Extending MkDocs
+
+---
+
+The MkDocs Developer Guide provides documentation for developers of third
+party themes and plugins. Please see the [Contributing Guide] for information
+on contributing to MkDocs itself. You can jump directly to a page listed
+below, or use the *next* and *previous* buttons in the navigation bar at the
+top of the page to move through the documentation in order.
+
+- [Themes](themes.md)
+- [Translations](translations.md)
+- [Plugins](plugins.md)
+- [API Reference](api.md)
+
+[Contributing Guide]: ../about/contributing.md