feat: add readthedocs config

[rgraber]

Description:

Adds a readthedocs.yaml file to allow us to configure the readthedocs settings in the GH repo. In particular, sets the python version to 3.8 (default on readthedocs is 3.7, which is incompatible with some of the required packages).

Also adds a long_description_content_type to get rid of a warning. There are still a lot of autodoc warnings, and fixing them all will take a while. I set fail_on_warning to false for now since I do not think this should be held up on fixing all the autodoc warnings.

Issue:

https://github.com/openedx/edx-django-utils/issues/301

Dependencies:

List dependencies on other outstanding PRs, issues, etc.

Reviewers:

• [ ] tag reviewer

Merge checklist:

• [ ] All reviewers approved
• [ ] CI build is green
• [ N/A ] Version bumped
• [ N/A ] Changelog record added
• [ ] Documentation updated (not only docstrings)
• [ ] Commits are squashed

Post merge:

• [ N/A ] Create a tag
• [ ] Check new version is pushed to PyPi after tag-triggered build is finished.
• [ ] Delete working branch (if not needed anymore)