docs: add inline example of pca_variance_ratio

[marvinbuff]

Part of #1664

Add inline example for sc.pl.pca_variance_ratio().

Done with @t-a-m-i and @polychromech.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.26%. Comparing base (f70e681) to head (a139ffc).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3950   +/-   ##
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  Coverage   78.26%   78.26%           
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  Files         117      117           
  Lines       12633    12633           
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  Hits         9887     9887           
  Misses       2746     2746           

Flag	Coverage Δ
hatch-test.low-vers	77.59% <ø> (ø)
hatch-test.pre	77.22% <ø> (ø)

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Files with missing lines	Coverage Δ
src/scanpy/plotting/_tools/__init__.py	77.02% <ø> (ø)