Add `pre-commit`, `pre-commit` gh action, and update contribution guide

[aaraney]

Add pre-commit configuration, a gh ci action that enforces pre-commit rules, and update contribution guide.

See: https://pre-commit.com/index.html

Highly recommend viewing individual commits when reviewing this. I used pre-commit to remove trailing white spaces and add missing trailing newlines.

In the future I plan to add linting and formatting stages using ruff. This seemed a little overly disruptive for now.

[aaraney]

@hellkite500, just fixed the merge conflict. Should be good to merge this now. Thanks! :tada: