add link to kwant website

doc/templates/layout.html

 {% extends "!layout.html" %}
 {% set script_files = script_files + ["_static/togglediv.js"] %}
+{% block rootrellink %}
+    <li><a href="http://www.kwant-project.org/">
+        <img src="{{ pathto('_static/kwant_logo.png', 1) }}"
+        style="vertical-align: middle; margin-top: -4px" alt="Kwant logo"/></a>
+      {{ reldelim1 }}</li>
+    <li><a href="{{ pathto('index') }}">{{ shorttitle }}</a>{{ reldelim1 }}</li>
+{% endblock %}