Add note regarding SSH host key replication step

What does this MR do?

Add note explaining step 2 is highly recommended, but not required.

https://docs.gitlab.com/ee/administration/geo/replication/configuration.html#step-2-manually-replicate-the-primary-nodes-ssh-host-keys

A few weeks ago a user opened a ticket(Unable to locate ticket) asking if this step was required as they do not intend to use disaster recovery. The current description of Step 2. Manually replicate the primary node’s SSH host keys does not state this is not required for functionality.

Related issues

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• If applicable, update the permissions table.
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