Add separate configuration for MkDocs Insiders plugins (#5544)

## Summary

This PR adds a separate configuration file to enable us to turn on
[Insiders-only
plugins](https://squidfunk.github.io/mkdocs-material/insiders/getting-started/#built-in-plugins).

I've turned on the `typeset` plugin which ensures that the settings on
the left-hand navigation pane render as code:

<img width="1792" alt="Screen Shot 2023-07-05 at 6 27 20 PM"
src="https://github.com/astral-sh/ruff/assets/1309177/c93676dd-bb48-417a-9d3b-528bf001e9b7">

.github/workflows/ci.yaml

@@ -271,4 +271,4 @@ jobs:
       - name: "Check docs formatting"
         run: python scripts/check_docs_formatted.py
       - name: "Build docs"
-        run: mkdocs build --strict -f mkdocs.generated.yml
+        run: mkdocs build --strict -f mkdocs.insiders.yml