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">
