docs: Add main extensions page

docs/extensions.md

@@ -0,0 +1,9 @@
+# Extensions
+
+Griffe has an extension system that allows Python developers to enhance or modify the data that Griffe collects. To learn more about how to use or develop extensions, please see the [Extending APIs topic](guide/users/extending.md).
+
+We group extensions under three categories:
+
+- [built-in extensions](extensions/built-in.md): maintained directly within Griffe's codebase
+- [official extensions](extensions/official.md): maintained in separated repositories, by the authors/maintainers of Griffe
+- [third-party extensions](extensions/third-party.md): maintained by other developers

mkdocs.yml

@@ -55,6 +55,7 @@ nav:
     - Project architecture: guide/contributors/architecture.md
     - Coverage report: guide/contributors/coverage.md
 - Extensions:
+  - extensions.md
   - Built-in:
     - extensions/built-in.md
     - dataclasses: extensions/built-in/dataclasses.md
@@ -255,7 +256,6 @@ plugins:
       loading.md: guide/users/loading.md
       expressions.md: guide/users/navigating.md#expressions
       best_practices.md: guide/users/recommendations/python-code.md
-      extensions.md: guide/users/extending.md
       docstrings.md: reference/docstrings.md
       parsing_docstrings.md: guide/users/how-to/parse-docstrings.md
       try_it_out.md: playground.md