feat(blog): add file-based blog with dynamic slugs, MDX content and layout shell

- Introduced blog routing using Next.js App Router
- Implemented dynamic [slug] pages for blog posts
- Added MDX-based content loading via lib/posts
- Integrated shared TopBar layout with navigation
- Established clear content, lib and component separation

apps/public-web/app/(site)/about/page.tsx

@@ -0,0 +1,16 @@
+import { TopBar, BackLink } from "../../../components/shell/TopBar";
+
+export default function AboutPage() {
+    return (
+        <div>
+            <TopBar title="Info" left={<BackLink href="/" />} />
+
+            <main className="mx-auto max-w-3xl px-4 py-10">
+                <h1 className="text-2xl font-semibold">About</h1>
+                <p className="mt-3 opacity-80">
+                    Platzhalter. Hier kommt spaeter Brand-Info, Links, Kontakt, etc.
+                </p>
+            </main>
+        </div>
+    );
+}