feat: add g analytics (cairo-book#432)

Author: fricoben

book.toml

@@ -18,3 +18,4 @@ after = ["cairo"]
 git-repository-url = "https://github.com/cairo-book/cairo-book.github.io/"
 edit-url-template = "https://github.com/cairo-book/cairo-book.github.io/edit/main/{path}"
 playground.runnable = true
+google-analytics = "G-0ZJLBV4KE2"

theme/index.hbs

@@ -103,6 +103,16 @@
             html.classList.add("sidebar-" + sidebar);
         </script>
 
+        <!-- Google tag (gtag.js) -->
+        <script async src="https://www.googletagmanager.com/gtag/js?id=G-0ZJLBV4KE2"></script>
+        <script>
+        window.dataLayer = window.dataLayer || [];
+        function gtag(){dataLayer.push(arguments);}
+        gtag('js', new Date());
+
+        gtag('config', 'G-0ZJLBV4KE2');
+        </script>
+
         <nav id="sidebar" class="sidebar" aria-label="Table of contents">
             <div class="sidebar-scrollbox">
                 {{#toc}}{{/toc}}