Start some sort of one-stop documentation

[M4a1x]

In addition to the README.md files sprinkled around the filesystem, a more extensive, prose-like documentation would be interesting. Options I could find are:

• MkDocs with mkdocstrings
• Sphinx
• Github wiki
• Static site generator, e.g. Hugo

Sphinx vs MkDocs In either case probably use RTD (readthedocs) theme.

Criteria:

• Simple to write (preferably Markdown)
• Ability to document code (but no priority, it's more about the whole project)
• Should be inside the git repo, no DB (e.g. Mediawiki). I want all in one place as much as possible. Are subrepos acceptable?
• No need to be editable by everyone PRs are fine.