doc: add missing ARIA label for button

The button for toggling light mode and dark mode has no text display.
Screen readers will read it as simply "button", making it not useful.
Add an aria-label attribute so it gets a better description.

PR-URL: https://github.com/nodejs/node/pull/37031


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>