Multi-level sidebar

[cr1315]

What would you like to be added:

Thank you for creating this theme, I really like the simple design and beautiful color schema, and would like to use this to create some guidelines for my project members.
But I found the sidebar could only support two level menus, it would be really great if it could support at least 4 or 6 level of menus, and much much better if each parent level is foldable !

I've searched the source and found that the menu level was hard-coded at useSidebar.js, and if that changed, something else like style and Sidebar component should also follow.
It would be my last choice to mangle it by myself, or, use another theme..
Before that, I would like to have an ask if you're willing to add such a feature, sincerely looking forward to it.

Why is this needed:

Well, it's just a need..

Is your enhancement request related to a problem? Please describe.

Additional context