Update docs

Author: Hexagon

docs/_includes/head_custom.html

@@ -1 +1,9 @@
-<script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-9224018205432249" crossorigin="anonymous"></script>
+<script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-9224018205432249" crossorigin="anonymous"></script>
+<script async src="https://www.googletagmanager.com/gtag/js?id=G-873B7M42VH"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', 'G-873B7M42VH');
+</script>