feat: add release notes page with MDX content system

Implemented comprehensive release notes feature with:

- MDX-based content system for easy release note authoring
- Thematic sidebar navigation (Foundation, Features, Infrastructure)
- Auto-generated pages from MDX frontmatter
- Overview page with status filtering (completed/in_progress/planned)
- Detail pages with custom MDX components
- Mobile-responsive navigation (horizontal scroll tabs)
- Integration with timeline (link to detailed release notes)
- Added "Releases" link to header navigation

Technical implementation:
- next-mdx-remote for MDX parsing and rendering
- gray-matter for frontmatter extraction
- Custom MDX components for images, code blocks, typography
- Static site generation (SSG) for performance
- Template system for consistent release note structure

Files added:
- app/(marketing)/releases/ - Route structure and components
- content/nl/releases/ - MDX content files and index
- lib/mdx/releases.ts - MDX utility functions
- docs/specs/releasepage/ - Build plan documentation
- docs/templates/release-note-template.mdx - Content template

First release note: authentication.mdx (login, signup, password reset)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -20,8 +20,10 @@
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "framer-motion": "^12.23.24",
+    "gray-matter": "^4.0.3",
     "lucide-react": "^0.553.0",
     "next": "16.0.1",
+    "next-mdx-remote": "^5.0.0",
     "next-themes": "^0.4.6",
     "react": "19.2.0",
     "react-dom": "19.2.0",