Add Plausible analytics

Author: julienrf

_includes/headertop.html

@@ -54,3 +54,6 @@
 
     <!-- Algolia stylesheet -->
     <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/docsearch.js@2/dist/cdn/docsearch.min.css" />
+
+    <!-- Plausible Analytics -->
+    <script defer data-domain="docs.scala-lang.org,scala-lang.org" src="https://plausible.scala-lang.org/js/script.js"></script>