Fixes #105906

This is common syntax used for intra-doc links, so fixing it should help
with doc formatting.

This code uses a special `hideme` class anyway, so just style that.

Fixes #100994

This selector was added in c7312fba.
The bug can be seen at <https://doc.rust-lang.org/1.27.0/alloc/slice/trait.SliceIndex.html#foreign-impls>.

This rule was added to help with a `<table>` that was used for displaying the
function signature [src] lockup. That lockup was changed in
34bd2b84 to use flexbox instead, leaving this
selector unused (at least, for its original purpose).

We recently removed the "up here" arrows on item-infos, and adjusted
vertical spacing so that even without the arrow, it would be visually
clear which item the item-info belonged to. The new CSS styles for
vertical spacing only applied to toggles, though. This missed
non-toggled impl blocks - for instance, those without any methods, like
https://doc.rust-lang.org/nightly/std/marker/trait.Send.html#implementors.
The result was lists of implementors that were spaced too closely. This
PR fixes the spacing by making it apply to non-toggled impl blocks as
well.

This also fixes an issue where item-infos were displayed too far below
their items. That was a result of display: table on .item-info .stab.
Changed that to display: inline-block.

This slightly reduces the noisiness of doc pages, making them easier to
read.

And add a rustdoc-gui test confirming various header sizes.

Detects https://github.com/rust-lang/rust/issues/88545