Improve backup and restore documentation for operator vs template deployment and HA

The current documentation for backup & restore is useful for template and operator deployments. However: 

• The title refers only to template deployment: 3scale backup and restore using templates.
• If the deployment is HA, steps to back up and restore system-mysql and system|backend-redis don't apply. It should be done with the correct procedure for each DB. We could add a comment at the top of this section to let the user know that they should follow their DB solutions' documented steps to perform the same actions. i.e. "If you have deployed MySQL extrenally please refer to <link to MySQL backup docs>".
• The restore subchapter differentiates between template and operator deployments. Maybe we could apply a similar structure to the backup subchapter.

It would be helpful to distinguish from the main chapter operator vs template deployments as well as point out the steps in case the DBs are externalised for HA.