Add ml inference ingest processor documentation

[mingshl]

Description

Add ml inference ingest processor documentation

Issues Resolved

Closes https://github.com/opensearch-project/documentation-website/issues/7041

Checklist

• [ ] By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and subject to the Developers Certificate of Origin. For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[hdhalter]

Thanks so much, @mingshl! Do you need a tech review or is it ready for doc review? In the meantime, can you please see if you can address some of the Vale style errors?

[mingshl]

@hdhalter addressed Vale style error. It's ready for tech review. Thanks!