[New] Manage Replication and Failover on a PostgreSQL Cluster Using r…

…epmgr (#6689)

* Initial draft of how to configure PostgreSQL replication using repmgr guide

* Tech Edit 1

* Updated front matter

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Co-authored-by: Adam Overa <adamovera@protonmail.com>
Co-authored-by: Matthew Wildman <matt@wildman.online>