Geo: Document GitLab site migration use-case

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Many customers have used Geo to migrate GitLab to a new architecture. This is done via normal replication and then planned failover, however there are some considerations specific to this use-case. It would be useful to have a doc describing how to approach this use-case.

Also, there are often many complex questions during the planning phase. It would be useful to group the answers in this doc.

Also, we could link such a doc from this line in the Planned failover doc.