* add pre-commit hooks for check & lock
* add pre-commit hook for export requirements.txt
* add pre-commit hooks to docs
* add pre-commit hook to check pre-commit-hooks.yaml file

Thanks @graingert and @asottile for pointing out the `files` constraint.
See discussion on #2511.

Co-authored-by: Thomas Grainger <tagrain@gmail.com>
Co-authored-by: Bjorn Neergaard <bjorn@neersighted.com>