Put docs in .gitignore.

[andy-hanson]

This is generated code, so it clutters PRs (that remember to run npm run verify to update docs), and can potentially get out-of-sync with the source (e.g. #1947, which has correct docs but a typo in src).
It also causes merge conflicts when docs/_data/rules.json changes.

[nchen63]

If we did this, only docs/_data/rules.json and docs/rules/* should be ignored. Also, if we ignored, we could no longer use /docs on master for gh-pages

[nchen63]

just ignoring rules.json for merge conflicts. I think it's still valuable to be able to serve /docs to gh-pages