[DOC] Star tree index feature documentation

[bharath-techie]

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.
Star tree index is a new feature that will be released in experimental mode to speed up the performance of aggregations by precomputing the results.

The users can provide star tree index mapping as part of index creation and users will be able to leverage the performance gains through star tree index while performing query. In 2.17, we will start with supporting metric aggregations with no filter and with term filter.

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

Edit : 2.18 [ Moved from 2.17 ]

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
opensearch-project/OpenSearch#13875

[bharath-techie]

Moved to 2.18