Reduce total size of offline html

[LuisPeregrina]

The generated docs are 350mb, I wondered why and it looks like the menu gets added to all html, which is about a megabyte.
This seems something that can be fixed with iframe or some other method since it uses Sphinx.

[Calinou]

This seems something that can be fixed with iframe or some other method since it uses Sphinx.

iframes cause many performance and usability issues when used for menus or sidebars. Modern websites avoid using iframes for UI elements because of this, reserving them to external embeds instead (mostly videos).

There are already other avenues that could be explored to reduce file size, as described in #11810.