docs: add inline example plot to sc.pl.rank_genes_groups_stacked_violin

[Ekin-Kahraman]

Part of #1664. Converts existing doctest to rendered plot showing top marker genes per group on PBMC 68k reduced.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.62%. Comparing base (1f9b5ca) to head (ef641ad).
⚠️ Report is 8 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4057   +/-   ##
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  Coverage   78.62%   78.62%           
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  Files         117      117           
  Lines       12727    12727           
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  Hits        10006    10006           
  Misses       2721     2721           

Flag	Coverage Δ
hatch-test.low-vers	77.92% <ø> (ø)
hatch-test.pre	78.50% <ø> (ø)

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Files with missing lines	Coverage Δ
src/scanpy/plotting/_tools/__init__.py	76.17% <ø> (ø)

[flying-sheep]

Thanks! This one doesn’t look right because of a seaborn bug, see #3991: https://icb-scanpy--4057.com.readthedocs.build/en/4057/api/generated/scanpy.pl.rank_genes_groups_stacked_violin.html