Document the migration for an existing HBase installation #142

As a user of an existing HBase installation I want to have documentation on how to migrate to Stackable.

Assume that I'm running on a non-containerized on-prem HDFS & HBase installation.

This is done when

• documentation is available on the steps needed to migrate an existing HBase 1.1.2 (HDP 2)
• documentation is available on whether (and if so) a rollback is possible and if so which data I need to backup
• Optional: Same but for these versions: HBase 1.2.0 (CDH 5), HBase 2.0.2 (HDP 3), HBase 2.1.4 (CDH 6), HBase 2.2.3 (CDP 7)

This should be in the form of a How-to guide as outlined here: https://diataxis.fr/how-to-guides/