docs(plotting): add inline plot example for highest_expr_genes

[haoyu-haoyu]

Summary

Add .. plot:: directive examples to the docstrings of two plotting functions that were missing inline examples:

• sc.pl.highest_expr_genes: default usage and n_top=10 variant
• sc.pl.highly_variable_genes: default usage and log=True variant

These are two of the remaining unchecked items from #1664.

Pattern

Follows the established pattern from existing inline examples (e.g., pca_loadings, rank_genes_groups_heatmap):

• First block loads data with sc.datasets.pbmc3k_processed()
• Subsequent blocks demonstrate parameter variations
• Uses :context: close-figs directive option

Ref #1664

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.06%. Comparing base (c7c1f4e) to head (4e8211f).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4025      +/-   ##
==========================================
- Coverage   78.51%   78.06%   -0.46%     
==========================================
  Files         117      117              
  Lines       12753    12753              
==========================================
- Hits        10013     9955      -58     
- Misses       2740     2798      +58     

Flag	Coverage Δ
hatch-test.low-vers	77.80% <ø> (ø)
hatch-test.pre	68.72% <ø> (-8.75%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/scanpy/plotting/_qc.py	86.11% <ø> (ø)

... and 2 files with indirect coverage changes

[flying-sheep]

hi, thank you for your contribution! @Ekin-Kahraman had the same idea for hvg, but uses the usual preprocessing, so I’ll merge both PRs.