docs: set up initial Sphinx configuration

[riebl]

I have played around a little bit with Sphinx to improve the documentation of our project (#9 I hear you).

Obviously, many things are still missing:

• [ ] Coverage of all our (public) macros
• [x] Automatic documentation rebuilds
• [x] Publish as GitHub page

[thor]

That's pretty cool, @riebl! I had a brief look around pt. 3, and I wonder if a Github action workflow that automatically builds the site and pushes it to gh-pages could be workable.

[riebl]

Thanks for the GitHub workflow hint! Before automation, we just need docs that look less like a construction site ;-)