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

[CuiweiG]

Part of the inline-example effort in #1664. Adds a rendered .. plot:: to sc.pl.scatter's docstring: a QC scatter (total counts vs detected genes, colored by mitochondrial fraction) on pbmc68k_reduced, which already carries the three required .obs columns so no preprocessing is needed.

Local verification

• pre-commit run --files src/scanpy/plotting/_anndata.py — all hooks pass
• pytest tests/test_plotting.py -k scatter — 35/35 passed
• Manually ran the example with show=False against current main; no errors

Refs #1664

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.60%. Comparing base (9909599) to head (bbd551c).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4078   +/-   ##
=======================================
  Coverage   78.60%   78.60%           
=======================================
  Files         118      118           
  Lines       12755    12753    -2     
=======================================
- Hits        10026    10025    -1     
+ Misses       2729     2728    -1     

Flag	Coverage Δ
hatch-test.low-vers	77.91% <ø> (+<0.01%)	⬆️
hatch-test.pre	78.49% <ø> (+<0.01%)	⬆️

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

Files with missing lines	Coverage Δ
src/scanpy/plotting/_anndata.py	85.42% <ø> (ø)

... and 7 files with indirect coverage changes

[CuiweiG]

Hi @flying-sheep, I just pushed a release note fragment to fix the validation check. Could this also be assigned to a milestone when you have a moment? Thanks!