Left ToC too narrow for nested sections and left body margin too small

[chrisjsewell]

This probably duplicates some other issues, but err.
Basically, I was looking into moving the aiida documentation from pydata-sphinx-theme to sphinx-book-theme (plus sphinx-panels -> sphinx-design)

and well here is a comparison:

honestly, I think I'll stick with the pydata-sphinx-theme for now, because it just looks "cleaner": it feels less crowded, and the body content is more towards the center of the screen