Add recent release changelog

[mdroidian]

Summary

• add CHANGELOG.md with the recent 1.8.1 and 1.8.0 releases
• document the project versioning cadence in the changelog intro

Test

• not run; docs-only change

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[charliecreates]

Reviewed the latest changes, and I don't have actionable feedback to address.