Add "back" announcement for screenreader users

[brea11y]

On November 14, 2024, Tracy Ferro (the National Secretary for the Blinded Veterans Association) requested the following update to the "back" link on our feature / child screens:

the “Back” button for going backwards in the app. I know the location is in the top left corner and listen for “Back”. It reads aloud the previous tab, ie. “Benefits, Link”. Took me a while to understand this is the back button. Can you make it announce “Back” as that is what we listen for?

Rather than updating every single child screen's visible copy to "back", I propose that we add an accessibility label to read out prior to the "back" link's copy. For example, the screenreaders should announce "back to benefits, link" or "back to health, link"

AC:

• Add an accessibility label of "back to" that reads out before the current / existing readout on the child screen links
• Send to @brea11y for review