[DOC] Document new experimental ingestion streaming APIs

[reta]

What do you want to do?

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

The new experimental ingestion streaming APIs are feature complete and need API specs.

The APIs use NDJSON (Newline delimited JSON, https://github.com/ndjson/ndjson-spec) which fits perfectly to the streaming use cases: each chunk sent along the request represents distinct bulk operation/action (index, create, delete, and update).

See please: opensearch-project/OpenSearch#9070

It is on schedule for 2.17.0 release.