Geo: Simplify the demotion process
## Summary
Demoting is essentially re-adding everything. We need better docs and should avoid rsyncing as much as possible. This is important for full failback.
## Ideas
* Single-command to demote a primary to a secondary.
* Support/require Patroni for single-node Geo deployments to avoid the need to completely resync the database and allow easier scale-out.
* Create runbook style documentation for single and multi-node Geo deployments.
* ...
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