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>

lib/mdx/releases.ts

@@ -0,0 +1,122 @@
+/**
+ * Release Notes MDX Utilities
+ *
+ * Functions for loading and parsing release note MDX files
+ */
+
+import fs from 'fs'
+import path from 'path'
+import matter from 'gray-matter'
+import { MDXRemote } from 'next-mdx-remote/rsc'
+
+const RELEASES_DIR = path.join(process.cwd(), 'content/nl/releases')
+
+export interface ReleaseFrontmatter {
+  title: string
+  category: string
+  group: 'foundation' | 'features' | 'infrastructure'
+  version: string
+  releaseDate: string
+  status: 'completed' | 'in_progress' | 'planned'
+  description: string
+}
+
+export interface ReleaseNote {
+  slug: string
+  frontmatter: ReleaseFrontmatter
+  content: string
+}
+
+/**
+ * Get all release note files
+ */
+export async function getAllReleases(): Promise<ReleaseNote[]> {
+  const files = fs.readdirSync(RELEASES_DIR)
+
+  const releases = files
+    .filter(file => file.endsWith('.mdx') && !file.startsWith('_'))
+    .map(file => {
+      const slug = file.replace('.mdx', '')
+      const filePath = path.join(RELEASES_DIR, file)
+      const fileContent = fs.readFileSync(filePath, 'utf-8')
+      const { data, content } = matter(fileContent)
+
+      return {
+        slug,
+        frontmatter: data as ReleaseFrontmatter,
+        content,
+      }
+    })
+
+  // Sort by group order and then by category
+  return releases.sort((a, b) => {
+    const groupOrder = { foundation: 1, features: 2, infrastructure: 3 }
+    const aOrder = groupOrder[a.frontmatter.group]
+    const bOrder = groupOrder[b.frontmatter.group]
+
+    if (aOrder !== bOrder) return aOrder - bOrder
+    return a.frontmatter.category.localeCompare(b.frontmatter.category)
+  })
+}
+
+/**
+ * Get a single release by slug
+ */
+export async function getRelease(slug: string): Promise<ReleaseNote | null> {
+  const filePath = path.join(RELEASES_DIR, `${slug}.mdx`)
+
+  if (!fs.existsSync(filePath)) {
+    return null
+  }
+
+  const fileContent = fs.readFileSync(filePath, 'utf-8')
+  const { data, content } = matter(fileContent)
+
+  return {
+    slug,
+    frontmatter: data as ReleaseFrontmatter,
+    content,
+  }
+}
+
+/**
+ * Get releases grouped by their group (foundation, features, infrastructure)
+ */
+export async function getReleasesGrouped() {
+  const releases = await getAllReleases()
+
+  return {
+    foundation: releases.filter(r => r.frontmatter.group === 'foundation'),
+    features: releases.filter(r => r.frontmatter.group === 'features'),
+    infrastructure: releases.filter(r => r.frontmatter.group === 'infrastructure'),
+  }
+}
+
+/**
+ * Get category metadata from index file
+ */
+export interface CategoryMetadata {
+  slug: string
+  title: string
+  group: string
+  description: string
+  order: number
+}
+
+export interface GroupMetadata {
+  id: string
+  title: string
+  description: string
+  order: number
+}
+
+interface IndexData {
+  groups: GroupMetadata[]
+  categories: CategoryMetadata[]
+}
+
+export async function getCategoryMetadata(): Promise<IndexData> {
+  const indexPath = path.join(RELEASES_DIR, '_index.json')
+  const indexContent = fs.readFileSync(indexPath, 'utf-8')
+  return JSON.parse(indexContent)
+}