[DOC] Fix nonexisting document 'client/advanced/features/engine_pool'

[dnskr]

Why are the changes needed?

The PR fixes nonexisting document 'client/advanced/features/engine_pool' warning:

./kyuubi/docs/client/advanced/features/index.rst:19: WARNING: toctree contains reference to nonexisting document 'client/advanced/features/engine_pool' [toc.not_readable]

The document was deleted in the PR #7118.

How was this patch tested?

Checked that there is no warning anymoreduring the documentation build process.

make html

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

No

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (a7ce23c) to head (806addc).

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #7221   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files         696     696           
  Lines       43530   43530           
  Branches     5883    5883           
======================================
  Misses      43530   43530           

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[dnskr]

Merged to master(1.11.0)