fix join section with wip alert

Author: marco66colombo

join.html

@@ -9,6 +9,7 @@
     <div class="bg-light container-fluid">
         <div class="px-4 py-5 my-5 text-center">
             <h1 class="display-5 fw-bold">Join</h1>
+            <p class="lead text-muted mt-3">Work in progress.</p>
         </div>
     </div>
     {% include footer.html %}