Centralize push options documentation
Right now push options are scattered in two places:
- https://docs.gitlab.com/ee/user/project/merge_requests/#git-push-options
- https://docs.gitlab.com/ee/ci/yaml/#skipping-jobs
It seems to me push options should be centralized in one big table that's easily searchable. Google doesn't turn up these items as the first hit.
/cc: @marcel.amirault