Geo docs: more on database replication and TLS

What does this MR do?

A lengthy customer emergency helping them recover from a DR test identified that our docs could do with

• guidance on how to directly check database replication
• some TLS troubleshooting

GitLab team members can read more in the ticket

I'm not clear how our setup docs for Geo ensure that the secondary trusts all the certificates at the primary, in the scenario where there's a cluster on the primary. repmgr in this case.

The additional troubleshooting steps will help with this, I've stepped back from trying to document setting up the bundle in the setup docs.

I assume all new setups going forwards will be Patroni, and am unclear how the TLS bit is handled there, (or how the geo secondary tracks the leader for that matter.)

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Also, revisions to docs for Patroni database replication - not sure how it happened, but the instructions in the original revision weren't right.

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