add official Chrome feature demos

public/demo/modern-web-guidance/app.js

@@ -0,0 +1,16 @@
+const dialog = document.querySelector("#guided-dialog");
+const openDialog = document.querySelector("#open-dialog");
+const copyButton = document.querySelector("#copy");
+const commands = document.querySelector("#commands");
+
+openDialog.addEventListener("click", () => {
+  dialog.showModal();
+});
+
+copyButton.addEventListener("click", async () => {
+  await navigator.clipboard.writeText(commands.textContent.trim());
+  copyButton.textContent = "已复制";
+  setTimeout(() => {
+    copyButton.textContent = "复制命令";
+  }, 1200);
+});