Generate documentation from typing indicators

[ocaballeror]

When we move to typing for the next release, we should also find some system to generate documentation based on those indicators. That would allow us to clean up some docstrings that only have a "Returns" section to indicate the return type.

This Sphinx extension looks like it could do the job, but I haven't looked much into it:
https://github.com/agronholm/sphinx-autodoc-typehints