help: Add a class to the help index page

- So that the page appearance can be easily customized in CSS.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/help/templates/help_index.html

@@ -7,6 +7,8 @@
 {% load firstboot_extras %}
 {% load static %}
 
+{% block body_class %}help-page{% endblock %}
+
 {% block content-container %}
   <div class="container content-container">
     <h2>{% trans "Help" %}</h2>