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() and :context: close-figs following the pattern from merged contributions to the #1664 checklist.

Checklist item from #1664.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.51%. Comparing base (dbb5c58) to head (2aad5b8).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

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  Coverage   78.51%   78.51%           
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  Files         117      117           
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Flag	Coverage Δ
hatch-test.low-vers	77.80% <ø> (ø)
hatch-test.pre	77.47% <ø> (ø)

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

[flying-sheep]

Hi thank you for your contribution!

Please next time don’t delete the PR template, but fill it out instead.

also looks like @haoyu-haoyu had the same idea a bit earlier in #4025, but you use the right preprocessing. https://icb-scanpy--4032.com.readthedocs.build/en/4032/api/generated/scanpy.pl.highly_variable_genes.html