Docs: rename "Screen readers" helper page to "Visually hidden"

Docs: rename "Screen readers" helper page to "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`