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: https://github.com/release-engineering/iib/commit/5d53652234a4c18e2e63aa26a3c2e1145d6d5d8d https://github.com/release-engineering/iib/commit/f1874eb2c3059eb07c307af815ac2c0d883594c8

[tkdchen]

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