Update docstrings, run in CI

[mattwthompson]

The docstrings in the generators serve as de facto documentation, and a handful of them were significantly out of date. This was a mild frustration while trying to figure out how to hook into some of the internals.

This PR updates the docstrings to run stand-alone and also adds a call to pytest --doctest-modules to automate this check and trigger soft failures when they fall out of sync.

[codecov-commenter]

Codecov Report

Base: 76.92% // Head: 76.92% // No change to project coverage 👍

Coverage data is based on head (47554cb) compared to base (b12c2e8).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #240   +/-   ##
=======================================
  Coverage   76.92%   76.92%           
=======================================
  Files           4        4           
  Lines         845      845           
=======================================
  Hits          650      650           
  Misses        195      195           

Impacted Files	Coverage Δ
openmmforcefields/generators/system_generators.py	50.58% <ø> (ø)
...penmmforcefields/generators/template_generators.py	86.04% <ø> (ø)

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[mikemhenry]

This is great, thanks @mattwthompson