Add additional assets to header area

This also fixes the custom template to use "extrahead". According
to the official doc, this seems to be the correct class to inherit.

https://www.sphinx-doc.org/en/master/templating.html#working-with-the-builtin-templates

Signed-off-by: Fujimoto Seiji <fujimoto@ceptord.net>

Author: fujimotos

source/_templates/layout.html

@@ -1,6 +1,6 @@
 {% extends "!layout.html" %}
 
-{% block  header %}
+{% block extrahead %}
 {{ super() }}
 <!-- Global site tag (gtag.js) - Google Analytics -->
 <script async src="https://www.googletagmanager.com/gtag/js?id=G-7EP88Q2KZ1"></script>
@@ -11,4 +11,19 @@
 
   gtag('config', 'G-7EP88Q2KZ1');
 </script>
+<!-- Google Tag Manager -->
+<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
+new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
+j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
+'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
+})(window,document,'script','dataLayer','GTM-NXD8Q6J');</script>
+<!-- End Google Tag Manager -->
+{% endblock %}
+
+{% block content %}
+<!-- Google Tag Manager (noscript) -->
+<noscript><iframe src="https://www.googletagmanager.com/ns.html?id=GTM-NXD8Q6J"
+height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>
+<!-- End Google Tag Manager (noscript) -->
+{{ super() }}
 {% endblock %}