docs: update changelog guidance

[tisonkun]

Summary

• populate the Unreleased changelog with the user-visible REQ, T-Digest, and intersection changes since 0.4.0
• document how to scope, group, and write changelog entries in CONTRIBUTING.md
• require repository agents to apply the same changelog guidance

Changelog scope

• use the 0.4.0 tag as the comparison baseline
• describe the final user-visible behavior from feat(req): port of reqsketch-rs  #204 and perf(tdigest): reduce allocation overhead during compression #209 through feat: return Option from intersection to_sketch before first update #211
• consolidate the related T-Digest performance work from perf(tdigest): reduce allocation overhead during compression #209 and perf(tdigest): single centroid buffer #210
• exclude test-only test(hll): cover union rebuild regressions #206 and tooling-only chore: migrate HawkEye to v7 #207

Validation

• cargo x check
• cargo x test
• cargo x lint
• git diff --check