site: change Plausible analytics instance

the previous link was actually enabled for a very short period of
time.
We have no meaningful data.
This one should be better.

I can share screenshots of the stats, and the dashboard link to contributors.

_layouts/default.html

@@ -78,7 +78,7 @@ <h1 id="title-non-xs"><a href="{{ site.baseurl }}/">The Common Lisp Cookbook</a>
       }
     </script>
 
-    <script async defer data-domain="lispcookbook.github.io/cl-cookbook" src="https://plausible.io/js/plausible.js"></script>
+    <script async defer data-domain="lispcookbook.github.io/cl-cookbook/" src="https://stats.documents.design/js/plausible.js"></script>
 
   </body>
 </html>