Use Insiders version of mkdocs-material (#5540)

## 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.

.github/workflows/docs.yaml

@@ -13,12 +13,15 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-python@v4
+      - name: "Add SSH key"
+        uses: webfactory/ssh-agent@v0.7.0
+        with:
+          ssh-private-key: ${{ secrets.MKDOCS_INSIDERS_SSH_KEY }}
       - name: "Install Rust toolchain"
         run: rustup show
       - uses: Swatinem/rust-cache@v2
       - name: "Install dependencies"
-        run: |
-          pip install -r docs/requirements.txt
+        run: pip install -r docs/requirements-insiders.txt
       - name: "Copy README File"
         run: |
           python scripts/transform_readme.py --target mkdocs