[DOC]Create documentation for _rethrottle API

[kolchfa-aws]

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 and all versions that are affected.

 POST /_delete_by_query/<task_id>/_rethrottle
POST /_update_by_query/<task_id>/_rethrottle
POST /_reindex/<task_id>/_rethrottle

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

[Naarcha-AWS]

There seems to be some overlap with the Reindex API, as shown in the API spec: https://github.com/opensearch-project/opensearch-api-specification/blob/main/spec/schemas/_core.reindex_rethrottle.yaml .

Need to find more information about the overlap before moving forward.