Add optional "pause replication" step

What does this MR do?

This is to discuss whether we should recommend the same "Optional. Pause replication …" step that we recommend for Geo-GitLab upgrades.

The rational given in this doc below the next command snippet …

Wait for the primary database to finish upgrading before beginning the following step, so the secondary can remain ready as a backup.

… is essentially the same, so should we be more explicit here by adding the optional step and link (see diff)?

In any case, it seems clear at this point that pausing replication isn't technically required, correct?

Related issues

1. internal customer question

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workflowready for review