[docs] Add gtag

docs/site/layouts/partials/head/custom-head.html

@@ -26,6 +26,17 @@
 </noscript>
 <!-- /Yandex.Metrika counter -->
 
+<!-- Google tag (gtag.js) -->
+<script async src="https://www.googletagmanager.com/gtag/js?id=G-2XLLXRJ1KF"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', 'G-2XLLXRJ1KF');
+</script>
+<!-- /Google tag (gtag.js) -->
+
 <!-- Main page cards -->
 <style>
   .pros-card {