Geo support in Helm chart: prepare documentation

Summary

From #8 (closed)

Add documentation (to doc/advanced) for setting up Geo:

• Creation of databases (including replication)
• Replicating secrets between clusters
  • railsSecrets
  • shell-host-keys
• Replication of object storage (MVC using provider)
• Configuration of charts

Reminder to remove this as a documented limitation.

cc @brodock