docs: record catalog documentation updates in changelog

[bntvllnt]

Summary

• Record the merged documentation refresh in the Unreleased changelog.
• Describe the applicable README/API/LLM-index cleanup without altering historical releases or dates.
• CHANGELOG.md only; no version bump, canary-version change, runtime change or publication.

Verification

• Signed commit independently verified; clean isolated worktree.
• Independent review and verification passed: additive changelog-only diff, unchanged package versions/history, whitespace and changed-entry formatting checks.
• Relevant package checks passed during implementation. Whole-file formatting differences, where present, also exist on the base and are left untouched.

[greptile-apps]

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