Consistently use two words for force field in documentation

[Yoshanuikabundi]

Previously, force field was inconsistently rendered as either "forcefield" or "force field". With this PR, documentation, notebook examples and code comments are consistently rendered as two words. This PR also addresses a few incorrectly capitalised and spaced references to the Open Force Field Toolkit, as well as a typo I noticed while going through the docs.

It also adds the auto-generated API documentation folder, docs/api/generated, to the .gitignore file. This appears to have been an oversight as it is automatically generated when the docs are built and previously could be committed.

Resolves #112.

Not addressed in this PR:

• Force field is typically rendered correctly according to PEP8 naming rules for class names (ForceField), but incorrectly in variable names (forcefield rather than the correct force_field). Fixing this would constitute a breaking change to the API.
• File names are not changed to preserve URLs
• No regression tests are added. If we later add lints for documentation, one of them can be a spellchecker and we can make sure forcefield is not in the dictionary. Short of that, a regression test would have to be familiar with every syntax in the repository so that it knows how to ignore URLs, filenames, code examples, actual code etc.

PR Checklist:

• Tag issue being addressed
• Add tests
• Update docstrings/documentation, if applicable
• Lint codebase
• Update changelog

[codecov]

Codecov Report

Merging #838 (55b7f96) into master (5cbfaf2) will decrease coverage by 0.27%.
The diff coverage is n/a.

[j-wags]

This looks great! A few suggested changes based on markdown/rendering weirdness. But otherwise this is perfect.

Feel free to merge once you've committed the suggested changes, or if you'd like we can merge at the beginning of our next call.