Readthedocs build should fail on warnings

[rgraber]

In order to do this, we should set fail_on_warning: true in .readthedocs.yaml.
Before doing so, however, we need to investigate and resolve the numerous autodoc warnings from the build.

[robrap]

There is a related setting SPHINXOPTS = -W # fail on warnings in the Makefile, as seen here: https://github.com/openedx/open-edx-proposals/blob/8dfc826fc83870c247f22f4c55a3dada0f334c32/Makefile#L5.

[rgraber]

@robrap That's what I was using to test locally

[robrap]

Cool. And if we implement this ticket, noting that both changes should happen in parallel.