Add a collapsible shortcut

app/assets/javascripts/vue_shared/components/markdown/header.vue

@@ -226,6 +226,12 @@ export default {
           />
         </div>
         <div class="d-inline-block ml-md-2 ml-0">
+          <toolbar-button
+            :prepend="true"
+            tag="<details><summary>Click to expand!</summary></details>"
+            :button-title="__('Add a collapsible section')"
+            icon="chevron-right"
+          />
           <toolbar-button
             :prepend="true"
             tag="- "