docs(client): Add comprehensive docs for Engine Pool feature

[z1131392774]

The documentation for the Engine Pool feature was previously missing. This commit introduces a complete guide to help users understand, configure, and use this feature effectively.

This new document (engine_pool.md) covers:

• An overview of the feature's purpose and mechanism.
• Detailed descriptions of all configuration parameters (e.g., kyuubi.engine.pool.size).
• Clarification on configuration precedence.
• Step-by-step instructions and practical examples for setup.

Additionally, the file format has been updated from .rst to Markdown (.md) for consistency with other project documentation.

Why are the changes needed?

The Engine Pool is a key feature for improving concurrency and resource utilization, but its documentation was absent. This made it challenging for users to discover and leverage this functionality. This PR provides a complete guide to enable users to effectively use the Engine Pool feature.

How was this patch tested?

This is a documentation-only change. The following has been verified:

• The Markdown content renders correctly in GitHub's preview.
• The technical explanations and configuration parameters are accurate.

Was this patch authored or co-authored using generative AI tooling?

Yes. Generative AI tools were used in several stages of preparing this contribution:

• The initial draft of the documentation was written with the assistance of Claude.
• Revisions and suggestions were provided by Gemini.
• Commit messages and pull request descriptions were refined for clarity using Gemini.

Generated-by: Claude 4, Gemini 2.5 pro

[pan3793]

I suggest embedding the engine pool description into the existing Engine Share Level page since engine pool is one specific case of the subdomain utilization

[pan3793]

An additional comment about AI-generated comments/articles, I'm not against that, but it's usually too wordy for technical docs, please keep the words short and clear.

[z1131392774]

An additional comment about AI-generated comments/articles, I'm not against that, but it's usually too wordy for technical docs, please keep the words short and clear.

Thanks for the great suggestions. That makes total sense.

I'll embed the content into the existing page and make the wording more concise. Will push the update shortly.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (364f062) to head (4d34a5b).
Report is 10 commits behind head on master.

Additional details and impacted files

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  Coverage    0.00%   0.00%            
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  Files         697     700     +3     
  Lines       43214   43374   +160     
  Branches     5855    5873    +18     
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- Misses      43214   43374   +160     

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