docs: add stats tracking

docs/_templates/layout.html

 {% extends "!layout.html" %}
 {% set css_files = css_files + ['_static/style.css'] %}
+
+{% block footer %}
+    {{ super() }}
+
+    <script>
+      var _paq = _paq || [];
+      _paq.push(['trackPageView']);
+      _paq.push(['enableLinkTracking']);
+      (function() {
+        var u=(("https:" == document.location.protocol) ? "https" : "http") + "://37.187.23.96/tr/";
+        _paq.push(['setTrackerUrl', u+'ep']);
+        _paq.push(['setSiteId', 6]);
+        var d=document, g=d.createElement('script'), s=d.getElementsByTagName('script')[0]; g.type='text/javascript';
+        g.defer=true; g.async=true; g.src=u+'js'; s.parentNode.insertBefore(g,s);
+      })();
+    </script>
+    <noscript><p><img src="http://37.187.23.96/tr/ep?idsite=6" style="border:0" alt=""></p></noscript>
+{% endblock %}