docs: add benchmarking guide (benchstat workflow + pprof) and perform…

[NesterovYehor]

Fixes #8574

docs/benchmark: add benchmarking guide and perf PR checklist

Adds benchmark/README.md with:

• How to run micro-benchmarks consistently
• Baseline vs. candidate comparison using benchstat
• Quick pprof (CPU/mem) tips

CONTRIBUTING.md: link to the guide and require benchstat diff + env/flags for perf PRs.

RELEASE NOTES:
n/a

[linux-foundation-easycla]

• ✅login: NesterovYehor / name: Yehor Nesterov / (628a006)

The committers listed above are authorized under a signed CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.85%. Comparing base (8420f3f) to head (628a006).
⚠️ Report is 18 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8577      +/-   ##
==========================================
- Coverage   80.91%   80.85%   -0.06%     
==========================================
  Files         413      413              
  Lines       40751    40773      +22     
==========================================
- Hits        32972    32969       -3     
- Misses       6155     6173      +18     
- Partials     1624     1631       +7     

see 23 files with indirect coverage changes

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[NesterovYehor]

Maintainers: could you please apply the label Type: Documentation (this PR adds docs only)? Thanks!

[easwars]

I see that the CI is failing for TestResolverBadServiceUpdate_NACKedWithoutCache. We recently fixed this. Would you please rebase your PR to master. Thanks.