Branch workflows backend/frontend integration + docs
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=515801) </details> <!--IssueSummary end--> ### Summary Once the ~frontend (https://gitlab.com/gitlab-org/gitlab/-/issues/515799) and ~backend (https://gitlab.com/gitlab-org/gitlab/-/issues/515800) is done we should integrate the frontend with the backend We should also add/update the necessary documentation and make the UI available in the branch rules details page.
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