Update/fix docs builds

[mattwthompson]

There was a small cross-interaction between #168 and #172 which didn't show up until they were in the same branch. It seems that python.install.method: setuptools internally calls python seutp.py but python.install.method: pip calls pip install -e ..

https://docs.readthedocs.io/en/stable/config-file/v2.html#packages

I tested this with ace4ee9 in #175 and it works but don't want to just sneak it in to a separate PR. I still intend to merge this without review since it's scoped to already-approved changes.

Changes made in this Pull Request:

• Fix RTD config to use pip install instead of python setup.py

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[mattwthompson]

Docs appear to be building fine: https://readthedocs.org/projects/openff-nagl/builds/26919217/

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.10%. Comparing base (ae50e54) to head (5c134e2).
Report is 1 commits behind head on main.

Additional details and impacted files