Add content grouping for Google Analytics.

Author: loranallensmith

source/layouts/layout.erb

@@ -53,7 +53,9 @@ under the License.
       function gtag(){dataLayer.push(arguments);}
       gtag('js', new Date());
 
-      gtag('config', 'G-8KCYZ2CYMS');
+      gtag('config', 'G-8KCYZ2CYMS', {
+        'content_group' : 'WooCommerce REST API Docs',
+      });
     </script>
   </head>