### Background
Vector checkbox hack implementations generally use `visibility: hidden` to hide the elements when the checkbox is not checked. This removes these elements from the accessibility tree, rendering them inaccessible to screenreader users.
Because both the main menu and collapsed TOC have landmarks in the hidden content itself, this means the landmark disappears when the checkbox is not checked. This is mentioned with the main menu in T311294.
Instead we want to use `.ui-helper-hidden-accessible`or equivalent CSS to only visually hide the main menu and collapsed TOC
### Developer notes
Vector dropdown menus are generally entirely contained in a landmark, so they don't need to be changed.
### AC
[] Main menu and "Site navigation" landmark is still accessible to screenreaders when the menu is closed
[] TOC and "Contents navigation" landmark is still accessible to screenreaders when the menu is closed