Doc: Improve container registry replication documentation

The container registry can be replicated using object storage in the same way as LFS objects, uploads, and CI artifacts, but the process isn't clearly documented. The container registry section is also confusingly under the disaster recovery section rather than the general geo section.

Proposal

• Move the container registry link under object storage section and add a warning that this is the only way to replicate the container registry.

Links / references

https://docs.gitlab.com/ee/administration/geo/replication/index.html#replicating-the-container-registry https://docs.gitlab.com/ee/administration/container_registry.html#container-registry-storage-driver