Link to mypy wiki page from CONTRIBUTING.md (#50540)
Summary:
Addresses one of the documentation points in https://github.com/pytorch/pytorch/issues/50513 by making it easier to find our `mypy` wiki page. Also updates the `CONTRIBUTING.md` table of contents and removes some trailing whitespace.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50540
Reviewed By: janeyx99
Differential Revision: D25912366
Pulled By: samestep
fbshipit-source-id: b305f974700a9d9ebedc0c2cb75c92e72d84882a
