use GTM and GA analytics (#129)

* Update _config.yml

* Update head_custom.html

Author: gkorland

_config.yml

@@ -3,7 +3,7 @@ title: FalkorDB Docs
 description: The FalkorDB documentation
 logo: "/images/falkor-logo.png"
 favicon_ico: "/images/favicon.ico"
-ga_tracking: GTM-MBWB627H
+gtm_tracking: GTM-MBWB627H
 
 search_enabled: true
 color_scheme: dark

_includes/head_custom.html

@@ -2,3 +2,12 @@
 <script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.10.0/highlight.min.js"></script>
 <script>hljs.highlightAll();</script>
 
+{% if site.gtm_tracking != nil %}
+<!-- Google Tag Manager -->
+<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
+new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
+j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
+'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
+})(window,document,'script','dataLayer','{{ site.gtm_tracking }}');</script>
+<!-- End Google Tag Manager -->
+{% endif %}