Add Reo.dev tracking script to documentation (#3094)

docs/reo-tracking.js

@@ -0,0 +1,16 @@
+(function() {
+  var clientID = 'e1256ea7e23318f';
+  
+  var initReo = function() {
+    Reo.init({
+      clientID: clientID
+    });
+  };
+  
+  var script = document.createElement('script');
+  script.src = 'https://static.reo.dev/' + clientID + '/reo.js';
+  script.defer = true;
+  script.onload = initReo;
+  
+  document.head.appendChild(script);
+})();