[enh] generate navigation for help pages

searx/templates/simple/help.html

@@ -1,5 +1,12 @@
 {% extends 'simple/page_with_header.html' %}
 {% block title %}{{ page.title }} - {% endblock %}
 {% block content %}
+<ul class="tabs">
+{% for name, page in all_pages %}
+  <li>
+    <a href="{{name}}" {% if name == page_filename %}class="active"{% endif %}>{{page.title}}</a>
+  </li>
+{% endfor %}
+</ul>
 {{ page.content | safe }}
 {% endblock %}