Docs: fix build, use mdbook

[TerrorJack]

We used to use sphinx+recommonmark for building docs. Problem is, recommonmark is deprecated, and although new solutions for supporting markdown in sphinx exist, markdown has never been a first class citizen in sphinx land anyway.

This PR fixes the docs build:

• We switch to mdbook now. First class support for markdown, and the generated pages support automatic dark mode, which is good for my soring eyes
• The docs build is now just a plain nix build. Sans examples, which still needs to be fetched from s3 for now
• After docs build finishes, click the commit status icon, there'll be a "docs deploy" entry that links to netlify deploy URL. No need to manually find that URL in CI log

[nrnrnr]

@TerrorJack This happened a little fast for my taste. Next time you want to pull the rug out from under me, please alert first.

Also, a quick Google search showed at least four static-site generators based on pandoc. Auto-dark mode should be possible to add...