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`

site/content/docs/5.0/helpers/visually-hidden.md

@@ -0,0 +1,25 @@
+---
+layout: docs
+title: Visually hidden
+description: Use these helpers to visually hide elements but keep them accessible to assistive technologies.
+group: helpers
+---
+
+Visually hide an element while still allowing it to be exposed to assistive technologies (such as screen readers) with `.sr-only`. Use `.sr-only-focusable` to visually hide an element by default, but to display it when it's focused (e.g. by a keyboard-only user). Can also be used as mixins.
+
+{{< example >}}
+<h2 class="sr-only">Title for screen readers</h2>
+<a class="sr-only-focusable" href="#content">Skip to main content</a>
+{{< /example >}}
+
+{{< highlight scss >}}
+// Usage as a mixin
+
+.sr-only-title {
+  @include sr-only;
+}
+
+.skip-navigation {
+  @include sr-only-focusable;
+}
+{{< /highlight >}}