refactor: rename /releases route to /documentatie

Complete route rename from /releases to /documentatie to better reflect
the continuous build-in-public nature of the project (not formal releases).

Changes:
- Renamed app/(marketing)/releases/ → app/(marketing)/documentatie/
- Renamed content/nl/releases/ → content/nl/documentatie/
- Updated all internal links from /releases to /documentatie
- Updated middleware publicRoutes (/releases → /documentatie)
- Updated navigation.json link
- Updated build-timeline.tsx documentation link
- Updated all component href attributes
- Updated lib/mdx/releases.ts content directory path
- Updated sidebar component paths and checks
- Updated MDX documentation files with new routes

Files changed:
- app/(marketing)/documentatie/[category]/page.tsx (footer link)
- app/(marketing)/documentatie/page.tsx (card links)
- app/(marketing)/documentatie/components/release-sidebar.tsx (all links)
- app/(marketing)/components/build-timeline.tsx (documentation link)
- content/nl/navigation.json (header link)
- content/nl/documentatie/*.mdx (internal references)
- middleware.ts (publicRoutes)
- lib/mdx/releases.ts (RELEASES_DIR path)

This prevents future confusion between "releases" (versioned software releases)
and "documentatie" (continuous build documentation).

Old URL: /releases/[category]
New URL: /documentatie/[category]

The old /releases route now returns 404 as expected.

app/(marketing)/components/build-timeline.tsx

@@ -242,7 +242,7 @@ export const BuildTimeline = ({ data }: BuildTimelineProps) => {
 
               {/* Link to detailed documentation */}
               <a
-                href="/releases"
+                href="/documentatie"
                 className="inline-flex items-center gap-2 text-teal-600 hover:text-teal-700 font-medium text-sm transition-colors"
               >
                 <span>Bekijk uitgebreide documentatie</span>