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>

docs-ui/README.md

@@ -13,3 +13,28 @@ yarn start
 ## Deployment
 
 Deployments are done automatically when a PR is merged into the `master` branch. We host the website using Github pages.
+
+## Maintaining Component Changelogs
+
+After a `@backstage/ui` release, sync the component changelogs to keep documentation up-to-date:
+
+```bash
+yarn sync:changelog
+```
+
+This script:
+
+- Parses `packages/ui/CHANGELOG.md` for new versions
+- Extracts entries tagged with "Affected components: ..."
+- Updates `src/utils/changelog.ts` with new entries
+- Handles both component-specific and general package changes
+
+After running, review the changes in `src/utils/changelog.ts` and commit them.
+
+**Preview changes before writing:**
+
+```bash
+yarn sync:changelog:dry-run
+```
+
+Running this gives you a summary of what would be written, without actually adding or changing any files.