Container repositories sync requires manual action to enable it , make it more visible in docs

We have said that container sync should be enabled separately https://docs.gitlab.com/ee/administration/geo/replication/#replicating-the-container-registry. However, we should refer to this section from installation steps too as it can be surprising from someone that they don't work form the box.

Maybe it makes sense to also put this into the "Limitations" section.