perf: speed up page rendering and jekyll build process (#2034)

- Ensure inline scripts execute after the DOM has fully loaded.
- Use Rollup to bundle the theme-mode and Mermaid scripts, reducing the number of Jekyll include snippets.

_includes/analytics/matomo.html

@@ -1,14 +1,13 @@
 <!-- Matomo -->
-<script type="text/javascript">
-  var _paq = window._paq = window._paq || [];
-  _paq.push(['trackPageView']);
-  _paq.push(['enableLinkTracking']);
-  (function() {
+<script>
+  document.addEventListener('DOMContentLoaded', () => {
+    var _paq = (window._paq = window._paq || []);
+    _paq.push(['trackPageView']);
+    _paq.push(['enableLinkTracking']);
     var u="//{{ site.analytics.matomo.domain }}/";
     _paq.push(['setTrackerUrl', u+'matomo.php']);
     _paq.push(['setSiteId', {{ site.analytics.matomo.id }}]);
     var d=document, g=d.createElement('script'), s=d.getElementsByTagName('script')[0];
     g.type='text/javascript'; g.async=true; g.src=u+'matomo.js'; s.parentNode.insertBefore(g,s);
-  })();
+  });
 </script>
-<!-- End Matomo Code -->