[Story a11y] Move focus back to page's content after screen reader triggered navigation

Part of #32511

> Focus remains on navigation buttons when activated
> When activating the previous/next/replay buttons, focus remains on the button that the user just activated. As a result, a keyboard user (and, in particular, users of assistive technologies such as screen readers) will have to navigate backwards to reach the content of the newly shown page.
>
> We recommend moving focus programmatically to the start of the page every time navigation occurs.

Steps to reproduce:
Open any AMP story using voice over
Click the next-page button

Expected Results:
Focus should go back to the page's content.

Actual Results:
Focus stays on the next-button and the user has to go backwards for voice-over to read the page's content

cc @ampproject/wg-stories