♻️(demo) refactor Onboarding modal display

Here is a shorter way to achieve the same result as before.

apps/demo/src/Onboarding.tsx

@@ -2,15 +2,15 @@ import React from "react";
 import { Button, Modal, ModalSize, useModal } from "@openfun/cunningham-react";
 
 const Onboarding = () => {
-  const modal = useModal({ isOpenDefault: true });
+  const modal = useModal({
+    isOpenDefault: !sessionStorage.getItem("onboarded"),
+  });
 
   const handleClose = () => {
     sessionStorage.setItem("onboarded", "1");
     modal.close();
   };
 
-  if (sessionStorage.getItem("onboarded")) return null;
-
   return (
     <Modal
       size={ModalSize.MEDIUM}