add Google AdSense script tag for some monetization chores

overrides/main.html

@@ -8,6 +8,7 @@ SPDX-License-Identifier: MIT
 {% block extrahead %}
 <link rel="stylesheet" href="{{ 'assets/stylesheets/mkdocs-material-styling.css' | url }}">
 <script src="https://hypothes.is/embed.js" async></script>
+<script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-7086107380680684" crossorigin="anonymous"></script>
 {% endblock %}
 <!-- Staging build -->
 {% block announce %}