Patroni: Document backup/restore procedures
### Problem to solve
Include specific instructions on how to run gitlab backup task against a Patroni cluster.
### Further details
It seems we are bounded to the same limitations from https://docs.gitlab.com/ee/raketasks/backup_restore.html#backup-and-restore-for-installations-using-pgbouncer
### Proposal
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### Who can address the issue
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### Other links/references
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issue