[DOC] Add documentation about Reader Writer Separation

[vinaykpud]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request.

We want to add a documentation on RW separation to serve as a guidance to how to setup the cluster with this setting enabled and best practices. More on the RW separation is in this design: opensearch-project/OpenSearch#15237

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
3.0

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
opensearch-project/OpenSearch#15306