Add Fathom Analytics to v5 docs

site/layouts/partials/analytics.html

@@ -5,3 +5,4 @@
   ga('send', 'pageview');
 </script>
 <script async src="https://www.google-analytics.com/analytics.js"></script>
+<script defer src="https://cdn.usefathom.com/script.js" data-site="TAXRUMOF"></script>