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/google.html

@@ -1,7 +1,7 @@
 <!-- Global site tag (gtag.js) - Google Analytics -->
 <script defer src="https://www.googletagmanager.com/gtag/js?id={{ site.analytics.google.id }}"></script>
 <script>
-  document.addEventListener('DOMContentLoaded', function (event) {
+  document.addEventListener('DOMContentLoaded', () => {
     window.dataLayer = window.dataLayer || [];
     function gtag() {
       dataLayer.push(arguments);