feat: add example components and system pages for UI showcase

- Introduced HeaderExample component for header display.
- Created StyleGuideOne, StyleGuideTwo, and StyleGuideThree components showcasing various UI elements and layouts.
- Added system pages: ComingSoon, Forbidden, Maintenance, and NotFound for better user experience during different states.
- Updated index.ts to export new components for easier access.

packages/ui/src/components/system-pages/not-found.tsx

@@ -0,0 +1,28 @@
+export function NotFound() {
+  return (
+    <div className="flex min-h-screen items-center justify-center bg-white px-4">
+      <div className="w-full max-w-md text-center">
+        <p className="text-sm font-medium uppercase tracking-widest text-gray-400">404 Error</p>
+
+        <h1 className="mt-3 text-6xl font-extrabold text-gray-900">Page Not Found</h1>
+
+        <p className="mt-4 text-base text-gray-500">
+          Sorry, the page you’re looking for doesn’t exist or has been moved.
+        </p>
+
+        <div className="mt-8 flex justify-center gap-3">
+          <button
+            onClick={() => window.history.back()}
+            className="rounded-md border border-gray-300 px-5 py-2.5 text-sm font-medium text-gray-700 hover:bg-gray-100"
+          >
+            Go Back
+          </button>
+
+          <a href="/" className="rounded-md bg-gray-900 px-5 py-2.5 text-sm font-medium text-white hover:bg-gray-800">
+            Back to Home
+          </a>
+        </div>
+      </div>
+    </div>
+  );
+}