Docs: migrate from mkdocs to sphinx

[onerandomusername]

Description

Sphinx is more supported, and has more features than mkdocs. There is a wider infrastructure than mkdocs, and sphinx can do more build-time injections.

Specifically, sphinx can show a version selector and show docs of specific version for free¹

In order to keep our docs in markdown, we are able to use MyST with Sphinx to minimize the changes required to our documentation pages. If we don't want to maintain markdown docs, it is possible to convert them to reStructured Text and then commit them as rst files.

For themes, there is a community collection of some of the themes visible at https://sphinx-themes.org/. Personally, I'm a fan of the furo theme.

Footnotes

1. Our theme actually does support a version selector, but it is not fully free. ↩