Simple working example of backup and restore for system-postgres & system-oracle

New notes regarding to backup/restore of external database have been introduced in upstream documentation https://github.com/3scale/3scale-Operations/blob/master/docs/day-2-operations/Backups.adoc by pull request https://github.com/3scale/3scale-Operations/pull/5

Product documentation deserves relevant update.

https://github.com/3scale/3scale-Operations/blob/master/docs/day-2-operations/Backups.adoc covers system-mysql and zync (postgres). We need a simple working example of backup and restore for system-postgres & system-oracle.

Scenario: upgrade from OCP3.x to OCP4.x: backup 3scale, restore on OCP4