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>

app/(marketing)/releases/layout.tsx

@@ -0,0 +1,34 @@
+/**
+ * Releases Layout
+ *
+ * Layout for release notes pages with sidebar navigation
+ */
+
+import type { ReactNode } from 'react'
+import { getAllReleases, getCategoryMetadata } from '@/lib/mdx/releases'
+import { ReleaseSidebar } from './components/release-sidebar'
+
+interface ReleasesLayoutProps {
+  children: ReactNode
+}
+
+export default async function ReleasesLayout({ children }: ReleasesLayoutProps) {
+  const releases = await getAllReleases()
+  const metadata = await getCategoryMetadata()
+
+  return (
+    <div className="min-h-screen bg-slate-50">
+      <div className="max-w-[1400px] mx-auto">
+        <div className="flex">
+          {/* Sidebar - hidden on mobile, fixed on desktop */}
+          <ReleaseSidebar releases={releases} metadata={metadata} />
+
+          {/* Main Content */}
+          <div className="flex-1 lg:ml-64">
+            {children}
+          </div>
+        </div>
+      </div>
+    </div>
+  )
+}