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/fathom.html

@@ -2,6 +2,5 @@
 <script
   src="https://cdn.usefathom.com/script.js"
   data-site="{{ site.analytics.fathom.id }}"
-  defer>
-</script>
-<!-- End Fathom Code -->
+  defer
+></script>