chore(post): style the Quick Jump list

website/source/_includes/post/table-of-contents.html.twig

@@ -1,4 +1,8 @@
 {% if toc %}
+    <div class="p-6 my-6 border border-gray-300 dark:bg-gray-800 dark:border-gray-700">
+        <p>Quick jump:</p>
+
+        <div class="mt-2">
             <ul>
                 {% for title in toc %}
                     <li>
@@ -6,4 +10,6 @@
                     </li>
                 {% endfor %}
             </ul>
+        </div>
+    </div>
 {% endif %}