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

[Ekin-Kahraman]

Part of #1664. Adds rendered plot examples to sc.pl.pca_variance_ratio showing:

1. Default variance ratio plot on PBMC 3k
2. Log-scale variant

Follows the same pattern as #4032 (merged).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.11%. Comparing base (1f9b5ca) to head (3b82349).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4056      +/-   ##
==========================================
- Coverage   78.62%   78.11%   -0.51%     
==========================================
  Files         117      117              
  Lines       12727    12727              
==========================================
- Hits        10006     9942      -64     
- Misses       2721     2785      +64     

Flag	Coverage Δ
hatch-test.low-vers	77.92% <ø> (ø)
hatch-test.pre	67.84% <ø> (-10.66%)	⬇️

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

... and 2 files with indirect coverage changes