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

[Ekin-Kahraman]

Adds an Examples section to the sc.pl.highly_variable_genes docstring with two inline plot blocks:

1. Minimal pipeline: load PBMC 3k, normalise, log-transform, compute HVGs, plot
2. Same data with log=True to demonstrate the parameter

Uses sc.datasets.pbmc3k() (same dataset as sc.pp.calculate_qc_metrics example) and :context: close-figs following the pattern established in merged contributions to this checklist.

Checklist item from #1664.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.50%. Comparing base (1fbe008) to head (6b60935).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4030   +/-   ##
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  Coverage   78.50%   78.50%           
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  Files         117      117           
  Lines       12740    12740           
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  Hits        10001    10001           
  Misses       2739     2739           

Flag	Coverage Δ
hatch-test.low-vers	77.79% <ø> (ø)
hatch-test.pre	77.45% <ø> (ø)

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Files with missing lines	Coverage Δ
src/scanpy/plotting/_preprocessing.py	86.27% <ø> (ø)