Docs: Migrate Gitaly chart to external Gitaly service

What does this MR do?

We recommend in our docs that customers should not use the Gitaly chart in a production environment.

We should add some docs that instruct how to migrate from the Gitaly chart to an external Gitaly service.

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Why backup/restore as documented in this MR instead of using the repository storage moves API?

The repository storage moves API requires that all Gitaly servers are visible to each other. This is a problem because the underlying Gitaly pod in the Chart is only accessible within the cluster.

To keep things simple, I have opted for the backup/restore approach but it does mean there will be downtime.

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