[Changelog] Public changelog page

## Summary
Public changelog for releases and updates.

## Components
- Changelog entries with date, version, description
- Categories: features, improvements, fixes
- Link to full release notes
- RSS feed

## Data Source
- MDX files or GitHub releases API

## Acceptance Criteria
- [ ] All releases listed
- [ ] Categorization clear
- [ ] RSS feed available
- [ ] Links to detailed notes