Rollup merge of #87663 - GuillaumeGomez:rustdoc-brace-minus-brace, r=notriddle

Rollup merge of #87663 - GuillaumeGomez:rustdoc-brace-minus-brace, r=notriddle

Rustdoc accessibility: use an icon for the [-]/[+] controls

This is a reopening of #87207 with improvement for the way of generating the `background-image` CSS property.

I quote from the original PR:

> This way, we can show the plus and minus buttons on screens, while voice
> control will read off actual words "Collapse" and "Expand" instead of reading
> "open brace minus close brace" and "open brace plus close brace".

Part of #87059

r? ``@notriddle``