add plausible.io

Author: vchuravy

_layout/head.html

@@ -11,6 +11,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1">
   {{if hasmath}} {{insert head_katex.html }}     {{end}}
   {{if hascode}} {{insert head_highlight.html }}   {{end}}
+  <script defer data-domain="juliaparallel.org" src="https://plausible.io/js/plausible.js"></script>
   {{insert style.html}}
   {{isdef title}} <title>{{fill title}}</title>  {{end}}
   <!-- end custom head snippets -->