[DOC] New `_scale` API which supports `search_only` mode

[prudhvigodithi]

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.

• Related PR: Search only replicas (scale to zero) with Reader/Writer Separation OpenSearch#17299.
• Related Issue for search only mode with all the design and implementation details [RW Separation] [Feature Request] Scale to Zero (Indexing Shards) with Reader/Writer Separation. OpenSearch#16720.

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
3.0.0

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
This is part of OpenSearch reader writer separation architecture opensearch-project/OpenSearch#15237.

[prudhvigodithi]

I will add more details to this issue, just want to start with the issue creation.
Thanks
@vinaykpud @Naarcha-AWS