docs(plotting): add inline plot examples to highest_expr_genes and highly_variable_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 78.50%. Comparing base (1fbe008) to head (c7eaecc).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4025   +/-   ##
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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)
src/scanpy/plotting/_qc.py	86.11% <ø> (ø)