Add CodePen link to the solidification front tutorial

Author: nikoscham

index.html

@@ -694,6 +694,12 @@ <h2 id="tutorials"><a name="Tutorials"></a>Tutorials</h2>
         <a href="https://feascript.com/tutorials/solidification-front-2d-worker.html" class="badge badge-mt"
           >Browser (multi-threaded)</a
         >
+        <a href="https://codepen.io/FEAScript/pen/dPMxaLq" class="badge badge-codepen"
+          >CodePen<img
+            src="https://upload.wikimedia.org/wikipedia/commons/4/44/Icon_External_Link.svg"
+            alt="External Link Icon"
+            style="width: 16px; height: 16px; vertical-align: middle; margin-left: 2px; margin-bottom: 4px"
+        /></a>
       </li>
     </ul>