Update documentation build

[cthoyt]

Summary

This adds testing to make sure sphinx can build the docs properly (and also makes necessary updates)

• [x] Code passes all tests
• [x] Unit tests provided for these changes
• [x] Documentation and docstrings added for these changes

Changes

• Add sphinx build check to CI
• Move documentation build requirements into setup.py (remove docs/requirements_1.txt and update readthedocs.yml)
• Use sphinx-automod-api to document groups of related things (datasets + models)
• Make imports explicit
• Removes custom CSS for now (until it can be updated to work both on local builds and also RTD)
• Add doc8 and docstr-coverage test to make sure everything is documented :)

[benedekrozemberczki]

This is amazing @cthoyt! The undeployed doc build checks were a pain before. Thank you for taking the time!

[cthoyt]

This is definitely why it pays to have a project template that enforces this stuff from the beginning - it's a ton of busy work to get stuff back to working if you're not sure what broke it