Improve documentation for upgrading 3scale on OCP 3.11 to OCP 4.x

We need to improve the documentation for upgrading 3scale on OCP 3.11 to OCP 4.x.

The process for migrating 3scale from template to operator-based deployments is currently documented here. There are examples of users that successfully migrated following these instructions. Also, the Managed Services team has been able to migrate an install between clusters; RHMI to RHOAM (See doc with details).

Nevertheless, there are several manual steps and limitations that need to be considered. The request is to improve our documentation to include more details regarding this process.

Some considerations/missing points: 

• Users need to manually update all instances of the domain name in the DB. (Support team produced related documentation in these KCSs: KCS1 & KCS2) 
• no reference to switching DNS over to the new cluster for public endpoints. 
• No reference to define the scope of migration e.g. analytics data in Redis will not be migrated
• Differences between using built-in APIcast, compared to self-managed APIcast