Improve plugin description to clarify use cases

[s4chinjha]

Fixes #1196

Description

Improved the plugin description to better explain what it does and when to use it.

It now clearly mentions that the plugin provisions Jenkins agents using Docker containers and highlights benefits like isolated builds and dynamic scaling.

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Testing done

• Ran Jenkins locally using mvn hpi:run
• Checked that the updated description is visible in the plugin manager
• Verified the Docker link works

Since this is only a documentation change, no tests were needed.

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Screenshots

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Submitter checklist

• Opened from a feature branch
• PR title is clear
• Linked the issue
• Explained changes and testing

[MarkEWaite]

Thanks very much. Approved with two optional comments for your consideration.

Whether you include those optional changes or not, I'll plan to merge this within the next 48-72 hours.