Generate and publish documentation

[akaihola]

Black has Sphinx documentation auto-generated from the extensive README.md file. The documentation is then published on readthedocs.io.

We could do the same, but since our README.rst is ReStructuredText like Sphinx documentation by default, we could use the .. include:: directive with the start-after and end-before options to include parts of the README in correct documentation sections, instead of running custom code to extract the text.