[Agentic Search 3.3] Using External MCP Tools with Agentic Search

[rithin-pullela-aws]

Description

Adds a page to describe how to make Agentic Search use external MCP servers

Issues Resolved

Closes #11229

Version

List the OpenSearch version to which this PR applies, e.g. 2.14, 2.12--2.14, or all.

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

• 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.

[github-actions]

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[natebower]

LGTM