Adds documentation for the Engine Pool feature to the `Share Level Of Kyuubi Engines` page. The separate and now-redundant `engine_pool.rst` file has been deleted. ### Why are the changes needed? The `Share Level Of Kyuubi Engines` documentation previously lacked a section on the Engine Pool feature. ### How was this patch tested? ### 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 Closes #7118 from z1131392774/docs/engine-pool-feature. Closes #7118 61892a2 [z1131392774] restore unnecessary style changes 02b4c67 [z1131392774] ci: Re-trigger CI checks 60154e0 [z1131392774] fix with spotless 9f07007 [z1131392774] docs: remove redundant engine_pool.md bf586f7 [z1131392774] update with SERVER_LOCAL and Engine Pool 4d34a5b [z1131392774] fix format with spotless 9ed2e8a [z1131392774] docs(client): Add comprehensive docs for Engine Pool feature Lead-authored-by: z1131392774 <z1131392774@gmail.com> Co-authored-by: z1131392774 <156416015+z1131392774@users.noreply.github.com> Signed-off-by: Cheng Pan <chengpan@apache.org>