[Pull-based Ingestion] Add experimental pull-based ingestion page

[varunbharadwaj]

Description

This PR adds a new page for pull-based ingestion. Pull-based ingestion is a new experimental feature introduced in OpenSearch 3.0. We add related details, message format and API details in a single page for now. This will be moved to individual pages once the feature is GA.

Issues Resolved

Closes #9545

Version

3.0 and beyond (experimental feature as of now)

Frontend features

If you're submitting documentation for an OpenSearch Dashboards feature, add a video that shows how a user will interact with the UI step by step. A voiceover is optional.

Checklist

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[kolchfa-aws]

Thank you, @varunbharadwaj! Re: page placement, I don't think this topic's best location is Creating and tuning your cluster because this section deals with cluster configuration and performance. I would create a new top-level topic "Ingestion" and move the current "Ingest pipelines" under that topic. And I would place Pull-based ingestion in that topic as well so the new structure would be:
Ingestion
|__ Ingest pipelines
|__ Pull-based ingestion

What do you think, @varunbharadwaj, @msfroh, @andrross?