feat: add alt text description for screenreader-only preview image

src/en/components/screenreader-only/use-case.md

@@ -9,7 +9,7 @@ eleventyNavigation:
   otherNames: visually hidden, assistive text.
   description: The screenreader-only component is text information only accessible with assistive technologies.
   thumbnail: /images/common/components/preview-screenreader-only.svg
-  alt: TO DO
+  alt: One blue-grey line with a dotted line border stacked above two grey lines. The colour of all of the lines fades from the left to the right ends. A blue-grey sound icon is superimposed on the two bottom lines.
   tag: Experimental
   state: published
 translationKey: 'screenreaderonly'