register: Update Zulip production server installation link.

Tweaked by tabbott to also recommend Zulip Cloud.

templates/zerver/realm_creation_failed.html

@@ -18,8 +18,15 @@ <h1 class="center">{{ message }}</h1>
         <div class="padded-content">
             <div class="inner-content">
                 <p>{{ _('This server does not allow members of the public to create new organizations.') }}</p>
-                <p>{% trans %}Zulip is open source, so you can install your own Zulip server by following the instructions on
-                    <a href="https://www.zulip.org/">www.zulip.org</a>{% endtrans %}</p>
+                <p>
+                    {% trans %}
+                    Zulip is open source, so you
+                    can <a href="https://zulip.readthedocs.io/en/stable/production/install.html">install
+                    your own Zulip server</a> or create a new
+                    organization on
+                    <a href="https://zulipchat.com">Zulip Cloud</a>
+                    {% endtrans %}
+                </p>
             </div>
         </div>
     </div>