help: Show menu on smaller screens also

- Fixes issue #1185.

Signed-off-by: Johannes Keyser <johanneskeyser@posteo.de>
Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/modules/help/templates/help_index.html

@@ -20,6 +20,14 @@
 
 {% load i18n %}
 
+{% block submenu %}
+  {% if submenu %}
+    <div class="sidebar">
+      {% include "submenu.html" with menu=submenu %}
+    </div>
+  {% endif %}
+{% endblock %}
+
 {% block content %}
 
   <h2>{% trans "Help" %}</h2>