Document container registry deployment/migration failure scenarios

Lists and describes all the identified possible failure scenarios when deploying and migrating to the new version of the container registry for GitLab.com.

Please see the architecture blueprint and the gradual migration plan for additional context.

Preview of the current status: https://gitlab.com/gitlab-com/runbooks/-/blob/jdrpereira-master-patch-27106/docs/registry/migration-failure-scenarios.md

Related to gitlab-org/container-registry#375 (closed).