feat(docs-ui): add automated changelog sync from @backstage/ui

Adds automated synchronization of component changelogs from
@backstage/ui package to the documentation site, eliminating
manual changelog maintenance.

The sync script parses CHANGELOG.md using remark/unified, extracts
component-specific entries via "Affected components:" tags, and
generates per-version TypeScript files with preserved markdown
formatting. Includes GitHub API integration to automatically fetch
PR numbers for changelog entries.

Key features:
- Version comparison to sync only new releases
- Per-version file structure (changelogs/v0.2.0.ts, etc.)
- Markdown preservation via AST position slicing
- Component name mapping (PascalCase → kebab-case)
- Duplicate detection for idempotent syncing
- GitHub authentication via gh CLI or GITHUB_TOKEN
- Dry-run mode with preview

Usage:
```bash
yarn sync:changelog              # Sync new entries
yarn sync:changelog:dry-run      # Preview changes
```

Technical changes:
- Added scripts/sync-changelog.mjs (991 lines)
- Refactored changelog.ts to import per-version files
- Extracted types to separate types.ts file
- Added dependencies: @octokit/rest, unified (remark-parse via @mdx-js/mdx)
- Added documentation in README.md

Signed-off-by: Johan Persson <johanopersson@gmail.com>

packages/ui/README.md

@@ -16,3 +16,26 @@ yarn add @backstage/ui
 - [Backstage UI Documentation](https://ui.backstage.io)
 - [Backstage Readme](https://github.com/backstage/backstage/blob/master/README.md)
 - [Backstage Documentation](https://backstage.io/docs)
+
+## Writing Changesets for Components
+
+When creating changesets for component-specific changes, add component metadata to help maintain documentation:
+
+```markdown
+---
+'@backstage/ui': patch
+---
+
+Fixed size prop handling for Avatar component.
+
+Affected components: Avatar
+```
+
+**Guidelines:**
+
+- **Component names**: Use PascalCase as they appear in imports (Avatar, ButtonIcon, SearchField)
+- **Multiple components**: `Affected components: Button, ButtonLink, ButtonIcon`
+- **General changes**: Omit the metadata line (build changes, package-level updates)
+- **Placement**: The line can appear anywhere in the description
+
+The changelog sync tool will parse these tags and update the documentation site automatically.