[DOC] Introduce Ml Inference Search Request Extension

[mingshl]

What do you want to do?

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

Tell us about your request. Provide a summary of the request.
Propose introducing an ml_inference search extension that can be used alongside any search query. This extension would allow users to pass a flexible, large object containing various model input formats, making it adaptable to different models.

RFC:
opensearch-project/ml-commons#3286

Version: List the OpenSearch version to which this issue applies,

2.19 or later

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

PR: opensearch-project/ml-commons#3284