Add google analytics to docs site

Author: bhch

docs/_includes/base.html

@@ -93,6 +93,18 @@ <h1>{{ title | upcase }}</h1>
     });
 </script>
 
+{% if project.node_env == 'production' %}
+    <!-- Global site tag (gtag.js) - Google Analytics -->
+    <script async src="https://www.googletagmanager.com/gtag/js?id=UA-94030773-1"></script>
+    <script type="text/javascript">
+        window.dataLayer = window.dataLayer || [];
+        function gtag(){dataLayer.push(arguments);}
+        gtag('js', new Date());
+
+        gtag('config', 'UA-94030773-1');
+    </script>
+{% endif %}
+
 {% for link in scripts -%}
     <script type="text/javascript" src="{{ link | url }}"></script>
 {% endfor -%}