Replace Google Analytics for Plausible (#293)

Replace Google Analytics for Plausible to make our docs more
privacy-friendly.

doc/_templates/layout.html

@@ -2,18 +2,8 @@
 {% extends "!layout.html" %}
 
 {% block extrahead %}
-    <!-- Google Analytics tracking code -->
-    <script>
-      (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
-        (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new
-            Date();a=s.createElement(o),
-          m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
-                })(window,document,'script','https://www.google-analytics.com/analytics.js','ga');
-
-      ga('create', 'UA-38125837-1', 'auto', {'storage': 'none'});
-      ga('set', 'anonymizeIp', true);
-      ga('send', 'pageview');
-    </script>
+    <!-- Plausible analytics for anonymous usage statistics -->
+    <script defer data-domain="fatiando.org" src="https://plausible.io/js/plausible.js"></script>
 {% endblock %}