add rtd config, isolate docs deps

[mbiokyle29]

Description

As part of the readthedocs onboarding process, a .readthedocs.yaml config file is required at the repo root. It also suggests a separate requirements file for the docs requirements, so I moved all docs pip deps into a new file (and -r'ed it into the dev one).

Screenshots & Media

Testing

I manually tested that docs build locally using a clean venv with just the docs requirements. See screenshots.

[heuermh]

Works for me locally