[help] stop rendering documentation with Jinja2

To facilitate translation of the user documentation we move
the templating logic outside of the user documentation.

searx/templates/oscar/about.html

@@ -2,4 +2,5 @@
 {% block title %}{{ _('about') }} - {% endblock %}
 {% block content %}
 {{ help.about | safe }}
+{% include "__common__/aboutextend.html" ignore missing %}
 {% endblock %}