Deploy the documentation to read the docs

[mprahl]

Cachito uses RST docstrings, so we should take advantage of that by generating documentation from them using sphinx and publishing it to Read the Docs.

See the following commits in IIB that did this for inspiration:
release-engineering/iib@5d53652
release-engineering/iib@f1874eb

[tkdchen]

It would be good to include "Enable regular autobuild" as a step of this work.