Rebalance workflow for GSI needs to be documented

GSI rebalance workflow needs better documentation. Rebalance is a very important operation for an administrator managing GSI.

Right now there is some information but it is fragmented across different places:

https://docs.couchbase.com/server/6.0/learn/clusters-and-availability/rebalance.html#rebalancing-the-index-service

https://docs.couchbase.com/server/6.0/n1ql/n1ql-language-reference/index-partitioning.html#rebalancing

GSI Rebalance Design Document

https://docs.google.com/document/d/11ZxY77aDPN6jG5QAdW_97wOSOtlGtLLzKaLwZPwGwDM/edit?usp=sharing