enhance_github pages doc (AtsushiSakai#675)

Author: AtsushiSakai

docs/_templates/layout.html

@@ -7,7 +7,7 @@
         crossOrigin="anonymous"></script>
 {% endblock %}
 
-{% block footer %}
+{% block content %}
 {{ super() }}
 <script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-9612347954373886"
      crossorigin="anonymous"></script>