[DOC] Append only index feature documentation

[RS146BIJAY]

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.

Append only indices is a new feature introduced which restricts any forms of updates or deletes of the document on an OpenSearch Index. This feature is particularly helpful for features like star tree where we want to keep documents immutable.

The users can enable append only index setting during index creation which will restricts any forms of updates on that index.

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#12886