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.

_layouts/default.html

@@ -74,8 +74,12 @@ layout: compress
       {% include_cached notification.html lang=lang %}
     {% endif %}
 
-    <!-- JavaScripts -->
-    {% include js-selector.html lang=lang %}
+    <!-- Embedded scripts -->
+
+    {% for _include in layout.script_includes %}
+      {% assign _include_path = _include | append: '.html' %}
+      {% include {{ _include_path }} %}
+    {% endfor %}
 
     {% include_cached search-loader.html lang=lang %}
   </body>