fix(solutions-blog): added not-found page (#902)

cc: @leerob 

### Description

Added a custom "not found" page. The default one breaks the layout.

### Demo

Screenshot:

![Screenshot 2024-04-09 at 09 58
07](https://github.com/vercel/examples/assets/5707542/8b17b405-bff4-496b-bc03-f0d1dfab812f)

### Type of Change

- [ ] New Example
- [x] Example updates (Bug fixes, new features, etc.)
- [ ] Other (changes to the codebase, but not to examples)

solutions/blog/app/not-found.tsx

@@ -0,0 +1,12 @@
+export default function NotFound() {
+  return (
+    <section>
+      <h1 className="mb-8 text-2xl font-semibold tracking-tighter">
+        404 - Page Not Found
+      </h1>
+      <p className="mb-4">
+       The page you are looking for does not exist.
+      </p>
+    </section>
+  )
+}