[site] switch to mkdocs-material

[sunshowers]

TODO:

• [ ] Replace remaining examples in image or HTML form with ANSI-formatted blocks
• [x] Add a sitemap
• [x] Add redirects
• [x] Update build process

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.99%. Comparing base (d15a9e3) to head (d6b1ee3).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1528 +/- ## ======================================= Coverage 78.99% 78.99% ======================================= Files 76 76 Lines 18785 18785 ======================================= Hits 14840 14840 Misses 3945 3945 ```

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[sunshowers]

This is better enough that I'm just going to land this, and fix up the remaining bits later.