Fix hover effects in sidebar
The dark and ayu themes have a menu-like highlight on sidebar items. The light theme used to, but it was accidentally lost in the sidebar unification. The change brings back the hover effect in the light theme.
It also makes the hover effect apply consistently to all links in the sidebar, including headings.
It also simplifies the "In _path_" heading so it's one big link. The breadcrumbs are still readily available at the top of the page.
Note that a small number of headings are not linkified and so don't get the hover effect. That will be fixed with #92957.
Demo: https://rustdoc.crud.net/jsha/sidebar-hover/std/string/trait.ToString.html
r? `@GuillaumeGomez`
Fixes#93115
267776aa2c