Update agentic search UI for 3.4

[ohltyler]

Description

Updates documentation for Agentic Search UI for 3.4. Refreshes the screenshots, and adds more detailed content for each of the supported features the UI exposes.

Issues Resolved

Closes #11583

Version

3.4 only

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.

Screenshots / steps outlined in the updated doc.

Checklist

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