Update docs with new way to add issues (and remove them) to issue boards
Update the docs due to the changes introduced here: https://gitlab.com/gitlab-org/gitlab-ce/issues/26205
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Update the docs due to the changes introduced here: https://gitlab.com/gitlab-org/gitlab-ce/issues/26205