Docs for canary deployments vol 2
What does this MR do?
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/31270
Includes commits from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10859
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/31270
Includes commits from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10859