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`

scss/_helpers.scss

@@ -2,6 +2,6 @@
 @import "helpers/colored-links";
 @import "helpers/embed";
 @import "helpers/position";
-@import "helpers/screenreaders";
+@import "helpers/visually-hidden";
 @import "helpers/stretched-link";
 @import "helpers/text-truncation";

scss/helpers/_visually-hidden.scss

@@ -1,5 +1,5 @@
 //
-// Screenreaders
+// Visually hidden
 //
 
 .visually-hidden,