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

[Ekin-Kahraman]

Part of #1664. Shows group correlation matrix with dendrogram on PBMC 68k reduced.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.61%. Comparing base (1f9b5ca) to head (ba97474).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4059      +/-   ##
==========================================
- Coverage   78.62%   78.61%   -0.01%     
==========================================
  Files         117      117              
  Lines       12727    12727              
==========================================
- Hits        10006    10005       -1     
- Misses       2721     2722       +1     

Flag	Coverage Δ
hatch-test.low-vers	77.91% <ø> (-0.01%)	⬇️
hatch-test.pre	78.49% <ø> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
src/scanpy/plotting/_anndata.py	85.42% <ø> (-0.11%)	⬇️