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.

_javascript/modules/components/mode-toggle.js

@@ -0,0 +1,15 @@
+/**
+ * Add listener for theme mode toggle
+ */
+
+const $toggle = document.getElementById('mode-toggle');
+
+export function modeWatcher() {
+  if (!$toggle) {
+    return;
+  }
+
+  $toggle.addEventListener('click', () => {
+    Theme.flip();
+  });
+}