Document how to provide feedback to GitHub for CI/CD pipelines

GitHub can receive feedback about pipelines running on external CI/CD. We now have the first step to integrate GitHub and GitLab CI/CD, using mirroring, but we still don't have a proper GitHub service integration (see the plan in https://gitlab.com/gitlab-org/gitlab-ee/issues/3835).

We can write some documentation about manual feedback from within jobs, calling API manually.