add collapsible

collapsible.js

@@ -0,0 +1,17 @@
+document.addEventListener('DOMContentLoaded', function() {
+  var headers = document.querySelectorAll('h4');
+
+  headers.forEach(function(header) {
+    header.addEventListener('click', function() {
+      var nextElement = header.nextElementSibling;
+      if (nextElement) {
+        var collapsibleList = nextElement.querySelector('.playlist, .notes, .contents');
+        if (collapsibleList) {
+          header.classList.toggle('active');
+          collapsibleList.classList.toggle('active');
+        }
+      }
+    });
+  });
+});
+