Consider switching guide.fatcat.wiki from mdbook (rust) to mkdocs (python)

The main motivation would be an easier to read and navigate theme. We could write or port a theme to mdbook, but the docs are "just markdown" so it would also be easy to switch to mkdocs, which has a larger community and optional features/plugins.

Here's an example theme that is particularly readable; we'd want to host fonts ourselves on (eg) archive.org:

https://squidfunk.github.io/mkdocs-material/getting-started/

Start by reviewing how guide.fatcat.wiki currently builds and publishes its Markdown through mdBook, then compare the proposed MkDocs Material theme and its font-hosting requirements. Done means agreeing on the migration approach and scope, with the guide rendered through the chosen system and its navigation and readability improved.

Written by the indexing model from the issue text.