add GTM (#213)

docs/gtm.js

@@ -0,0 +1,5 @@
+(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','GTM-MDHLQC4');

mkdocs.yml

@@ -7,6 +7,9 @@ google_analytics:
     - 'UA-92003007-1'
     - 'auto'
 
+extra_javascript:
+    - gtm.js
+
 theme:
   name: 'material'
   language: 'en'