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).
✅ All tests successful. No failed tests found.

Additional details and impacted files

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

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

Flags with carried forward coverage won't be shown. Click here to find out more.

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