Update lint rules, and clarify docs around linting

Created by: ttmarek

@Timer I made a long-winded issue a couple of days ago about linting. I've closed that issue because it was mostly rant. Instead, I'm proposing some changes to the ESLint rules and docs that we can iterate on in this pull request.

1. I updated CRA's rules so they are all configured as warnings.
2. I removed the ESLint rules that are style-specific. I did this by looking at rules that we had that were in common with those in eslint-config-prettier.
3. I updated the docs to clarify how linting works in CRA, and to provide some tips on how to do certain things that I struggled with initially.