Clean up documentation and deploy automatically

[epretti]

Sets up GitHub Actions to automatically build and deploy the Sphinx documentation to GitHub Pages. This is mostly modeled after the way OpenMM-ML and MLIPOps do their documentation. It also moves a lot of material from the README that should have been in a user guide into an actual user guide.

I don't think this can easily be tested on this repository before we actually merge it to trigger the action. However, I've been testing it on my fork so the output can be previewed at https://epretti.github.io/openmmforcefields.

A gh-pages branch will also need to be set up so the action has somewhere to push to.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.48%. Comparing base (fb86b91) to head (5bd3af6).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #453   +/-   ##
=======================================
  Coverage   84.48%   84.48%           
=======================================
  Files           5        5           
  Lines         799      799           
=======================================
  Hits          675      675           
  Misses        124      124           

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