Clarify benchmark scope

[galjos]

Summary

• describe external comparisons as regression and method benchmarks
• document the shared thermochemistry core and nitric-oxide source caveat

Validation

• strict documentation build passes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.82%. Comparing base (c48c49f) to head (72e6d0d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #142   +/-   ##
=======================================
  Coverage   95.82%   95.82%           
=======================================
  Files          34       34           
  Lines        2877     2877           
=======================================
  Hits         2757     2757           
  Misses        120      120           

Flag	Coverage Δ
unittests	95.82% <ø> (ø)

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