Add some information in the docs about users with external databases and Cloud users.

Backups:

• Add a note to the Backup chapter that, for users with external databases, the automatic backup option or backing up using RHACS is not available. (If Central does not know about the database, it cannot control backups of it.)
• Suggest that users with external databases scale down Central before upgrading their database, because Central will attempt to connect to the database until it can successfully connect.
• Advise that users with external databases should use whatever backup procedures are recommended by the database vendor.
• Add note for Cloud users that the automated backup option or starting the backup process from an integration is not possible. (Red Hat is responsible for backing up Cloud customers' data.)