[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/simple/about.html

@@ -1,4 +1,5 @@
 {% extends 'simple/base.html' %}
 {% block content %}
 {{ help.about | safe }}
+{% include "__common__/aboutextend.html" ignore missing %}
 {% endblock %}