build: Adjust docs/conf.py to fix RTD build

[sarina]

See https://daniel.feldroy.com/posts/2023-01-configuring-sphinx-auto-doc-with-django for two issues - path needed to be specified, and DJANGO_SETTINGS_MODULE needs to be set after the path is fixed.

THE DOC BUILD... IT LIVES

[sarina]

@ormsbee the docs are absolutely now BUILDING on PRs: https://app.readthedocs.org/projects/openedx-core/builds/ but they are not reporting on them. I'll check with Feanil, I'm not sure what steps I may have missed.

I also dug in a bit to your question about headers. With napoleon - already installed on this repo - we could convert docstrings to Google Python or NumPy-style docstrings, with sections instead of :param ...:s. And a lot of header options are already built in. This won't necessarily solve all your header problems, but might be helpful.

[sarina]

I didn't have to bother Feanil, I remembered what I did wrong (didn't give the readtheddocs bot WRITE access to the repo - https://github.com/orgs/openedx/teams/bot-readthedocs)

@ormsbee this is ready to merge