docs: add instructions for running performance benchmarks

[dosubot]

Update documentation for #265

Updates the CONTRIBUTING guide with a new Benchmarks section. It explains that benchmarks in benchmarks/ are disabled by default via pytest’s --benchmark-disable, and provides the command to run them with pytest --benchmark-enable -n 0 benchmarks/. The note clarifies that -n 0 disables pytest-xdist parallelism to keep benchmark measurements accurate, and mentions the suite includes engine performance tests.

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