docs: document how to run JMH benchmarks

[zeitlinger]

Adds a ### Benchmarks section to CONTRIBUTING.md documenting how to run JMH benchmarks and what output to include in performance PRs.

Motivated by #8240, where the author ran local benchmarks but the repo had no guidance on the established JMH setup.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.29%. Comparing base (1e0ddc6) to head (1847932).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8271   +/-   ##
=========================================
  Coverage     90.29%   90.29%           
  Complexity     7656     7656           
=========================================
  Files           844      844           
  Lines         23071    23071           
  Branches       2311     2311           
=========================================
  Hits          20832    20832           
  Misses         1521     1521           
  Partials        718      718           

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