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/comment.html

@@ -0,0 +1,5 @@
+<!-- The comments switcher -->
+{% if page.comments and site.comments.provider %}
+  {% capture path %}comments/{{ site.comments.provider }}.html{% endcapture %}
+  {% include {{ path }} %}
+{% endif %}