Add ARIA attributes to tab buttons

This is about the buttons to toggle the pages in the live docs / status panel - which I also discussed in #3273. I decided to make this one a separate issue because you can tackle them independently.

From what I can see, these controls are <button> elements without any further semantics encoded (through a role or ARIA attributes). That indicates that you can push them, but there is no encoding that these buttons have an active/inactive status, or that they control a different part of the page. You can see these things visually, but it's not clear for assistive technologies.

Solution: the way these interactions are programmed matches the disclosure pattern. Set the aria-expanded and aria-controls attributes based on the state of the widget and this is good to go!