chore: add goatcounter

Author: gildesmarais

_includes/head_custom.html

@@ -15,3 +15,4 @@
 <meta name="msapplication-TileColor" content="#111111">
 <meta name="msapplication-config" content="/assets/images/browserconfig.xml">
 <meta name="theme-color" content="#111111">
+<script data-goatcounter="https://html2rss.goatcounter.com/count" async src="//gc.zgo.at/count.js"></script>