Link to the "Announce a deployment" how-to in the Runbooks
This addresses https://gitlab.com/gitlab-com/infrastructure/issues/1361 and depends on https://gitlab.com/gitlab-com/runbooks/merge_requests/304.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
This addresses https://gitlab.com/gitlab-com/infrastructure/issues/1361 and depends on https://gitlab.com/gitlab-com/runbooks/merge_requests/304.