Move documentation to mkdocs

[gsarti]

Description

While the current furo theme for Sphinx looks nice, we have several reasons to prefer mkdocs in the long term:

• mkdocs has out-of-the-box support for markdown, which would simplify the writing of docstrings and doc pages for new contributors not familiar with restructured text.

• mkdocs supports autodoc-like API documentation via mkdocstrings, including Google-style docstring support. This is quite important since the only solution to add markdown to Sphinx, i.e. MyST-Parser, does not support Google docstrings and no timeline is provided.

• Markdown support would enable visualizing links, style etc. in docstrings via inseq.explain using rich markdown support. This is not possible with the current restructured text format.

• Moving to markdown would also allow us to setup faqtory to start automating well-known question/issues.

Also, ReadTheDocs has out-of-the-box support for mkdocs, which would simplify the move.

Example of nice mkdocs designs:

• mkdocstrings/python: https://mkdocstrings.github.io/python/
• Textualize/textual: https://textual.textualize.io/