## Summary
This PR migrates our `mkdocs-material` version to
[Insiders](https://squidfunk.github.io/mkdocs-material/insiders/), which
we can access now that we're sponsors.
We can't allow public access to the Insiders version, so we instead have
a private fork, which contains a deploy key that I've added as a
read-only Actions secret in this repo. (That is: the deploy key only
lets you read that one repo, and do nothing else.)
In general, non-Astral contributors can use the non-insiders version,
and everything is expected to "work", but without the insiders features
(they're intended to be ignored). See:
https://squidfunk.github.io/mkdocs-material/insiders/#compatibility.
