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 90.27%. Comparing base (5f5b054) to head (a5871e8).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8271      +/-   ##
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+ Coverage     90.25%   90.27%   +0.01%     
- Complexity     7691     7692       +1     
============================================
  Files           850      850              
  Lines         23203    23207       +4     
  Branches       2356     2356              
============================================
+ Hits          20943    20950       +7     
+ Misses         1532     1530       -2     
+ Partials        728      727       -1     

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[jack-berg]

Similar content is available here: https://github.com/open-telemetry/opentelemetry-java/blob/main/docs/jmh.md

I like this location better. At least for now until we have a strategy to extend / break apart CONTRIBUTING.md. But can you delete the duplicate content so we have just one copy?

[jack-berg]

This will be superseded by #8303

[jack-berg]

Now lives in https://github.com/open-telemetry/opentelemetry-java/blob/main/docs/knowledge/other-tasks.md#benchmarks-jmh

This PR has added text about running benchmarks when proposing performance sensitive PRs, which would still be useful to add.