update join button behavior

Author: marco66colombo

index.html

@@ -151,7 +151,7 @@ <h1 class="display-5 fw-bold">Collaboration</h1>
                     <a href="{{ site.baseurl }}/collaboration.html" class="btn btn-primary btn-lg px-4 gap-3">Code of Conduct</a>
                 </div>
                 <div class="d-grid gap-2 d-sm-flex justify-content-sm-center mt-3">
-                    <a href="{{ site.baseurl }}/join.html" class="btn btn-primary btn-lg px-4 gap-3">Join</a>
+                    <a href="{{ site.contact_mailto }}" class="btn btn-primary btn-lg px-4 gap-3">Join</a>
                 </div>
                 <div class="pt-4 collab-block">
                     <h2 class="h3 fw-bold text-center">Collaborating Institutes</h2>