Document the upgrade steps for the Internal Postgres DB from version 10 to version 13

This guide will be for customers who wish to simply upgrade the existing internal DB from 10 to 13.
There will be a separate guide for customers who want to migrate to an external remote DB

This guide should be applicable to all customers from 2.12 onwards, as the first step should be
to mark the DB as external (self-managed) and this in turn will stop the reconciliation of the DB
by the Operator

scl guide