Add custom admonition for code sandbox (#95)

sandbox.js

@@ -0,0 +1,13 @@
+(() => {
+  const boxes = document.querySelectorAll("details.admonish-sandbox");
+  boxes.forEach((box) => {
+    const once = () => {
+      const template = box.querySelector("template");
+      const sandbox = template.content.cloneNode(true);
+      template.after(sandbox);
+
+      box.removeEventListener("toggle", once);
+    };
+    box.addEventListener("toggle", once);
+  });
+})();