Add google analytical links to docs.

Signed-off-by: Tao He <linzhu.ht@alibaba-inc.com>

docs/_templates/layout.html

@@ -7,6 +7,22 @@
 
 #}
 
+{%- block extrahead %}
+
+{{ super() }}
+
+<!-- Global site tag (gtag.js) - Google Analytics -->
+<script async src="https://www.googletagmanager.com/gtag/js?id=G-4TMYCGJ0X2"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', 'G-4TMYCGJ0X2');
+</script>
+
+{% endblock %}
+
 {% block navigation %}
   <div class="wy-menu wy-menu-vertical" data-spy="affix" role="navigation" aria-label="main navigation">
     {% block menu %}