docs: add benchmark baseline reporting guidance

[kholdrex]

Summary

Adds guidance for capturing and sharing reproducible Criterion benchmark baselines. The documentation emphasizes local comparisons and benchmark artifacts instead of flaky CI timing thresholds.

Changes

• Documented named Criterion baseline capture and comparison commands.
• Listed benchmark context to include in reviews, including git SHAs, Rust toolchain versions, system details, commands, and report locations.
• Clarified that CI should compile benchmarks without treating noisy timing deltas as pass/fail gates.
• Added an Unreleased changelog entry for the documentation update.

Test plan

• Rust formatting passed.
• Full Rust test suite passed with all features.
• Clippy passed with warnings denied.
• Documentation built with warnings denied.
• Benchmark targets compiled successfully.