[enh] introduce /help route

Translation will be implemented in the future.
For now the "en" in /help/en/<pagename> is hardcoded.

searx/templates/simple/help.html

@@ -0,0 +1,5 @@
+{% extends 'simple/page_with_header.html' %}
+{% block title %}{{ page.title }} - {% endblock %}
+{% block content %}
+{{ page.content | safe }}
+{% endblock %}