Derived field updates for 2.17

[mch2]

Description

2.17 Updates to Derived Fields documentation. This PR

1. Include agg support with an example
2. Remove concurrent search limitation

Issues Resolved

Resolves #7902
Resolves #7850

Version

2.17

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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[mch2]

When you're ready for doc review, tag the assignee of this PR. The doc reviewer may push edits to the PR directly or leave comments and editorial suggestions for you to address (let us know in a comment if you have a preference). The doc reviewer will arrange for an editorial review

tagging @kolchfa-aws I think this is good to go.

[mch2]

re-opening under diff branch