✨(demo) add app demo

This demo is useful to test in the real usage of the design system as a real
user that will generate its own customized design tokens, implements the design
system's components, etc ...

apps/demo/src/main.tsx

@@ -0,0 +1,13 @@
+import React from "react";
+import ReactDOM from "react-dom/client";
+import "./index.scss";
+import { Button } from "@openfun/cunningham-react";
+
+ReactDOM.createRoot(document.getElementById("root") as HTMLElement).render(
+  <React.StrictMode>
+    <div className="center">
+      <h1 className="test">Cunningham Demo.</h1>
+      <Button />
+    </div>
+  </React.StrictMode>
+);