* sr-only -> visually-hidden cleanup
It seems the old screen-readers.md file was left behind, and forgot to rename the mixin
* Fix broken mixins for visually-hidden
"screen readers" is quite reductive, as there are other assistive technologies. content hidden this way is even announced by things like Alexa/Siri etc, so it's not so much just "screen readers".
in the long run, we may even consider changing the actual classnames (maybe `.visually-hidden` / `.visually-hidden-focusable`, though admittedly that's a bit verbose).
also includes a tiny tweak to layout.md to generalise the note about using `.sr-only`
* Rename `sr-only`/`sr-only-focusable`
To be more representative of the fact that these are not necessarily "screen reader" specific, but actually apply to assistive technologies in general (and also things like Alexa/Siri/etc). Goes hand-in-hand with #31133
Co-authored-by: XhmikosR <xhmikosr@gmail.com>
