Document cluster scaling

[siegfriedweber]

Changing the replica count of nodes without data loss or changing the disk size of data nodes is not supported by the operator. It can be done manually but the steps are not trivial and must be documented.

Part of #1

Documentation

https://docs.stackable.tech/home/nightly/opensearch/usage-guide/scaling/

Tasks

• Add the role-group as a node attribute and document cluster scaling #63