Add Google Analytics code for knowing the statistic of site

Author: donho

themes/book/layouts/partials/docs/inject/head.html

@@ -0,0 +1,9 @@
+<!-- Global site tag (gtag.js) - Google Analytics -->
+<script async src="https://www.googletagmanager.com/gtag/js?id=UA-151884174-1"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', 'UA-151884174-1');
+</script>