New documentation

First commit, halfway through a major restructuring of documentation.

docs/_includes/google_analytics.html

@@ -0,0 +1,6 @@
+<!-- the google_analytics_id gets auto inserted from the config file -->
+
+{% if site.google_analytics %}
+
+<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','//www.google-analytics.com/analytics.js','ga');ga('create','{{site.google_analytics}}','auto');ga('require','displayfeatures');ga('send','pageview');</script>
+{% endif %}