Improve content structure initial focus

[afercia]

This PR tries to solve the last pending issue from #3216. Initial focus on the "Content Structure" Popover needs to take into account post without any headings.

• when there are no headings, this Popover has no focusable elements to move focus to
• when there are headings, focus is currently moved to the first item in the headings tree; however, this way all the previous content (the "counts") is basically "skipped" and screen reader users would have no clue some content exists before the focus

By making the "counts" section a focusable div, with a proper role and aria-label, the first focusable element in this Popover is always the counts section.

• note: I'm fine with having no focus style in this case, it's a trade-off for a greater good
• about the note role, see https://www.w3.org/TR/wai-aria-1.1/#note

A section whose content is parenthetic or ancillary to the main content of the resource.

Seems appropriate to me, can certainly be changed if anyone suggests a better role.

[jorgefilipecosta]

Changes seem good to me. And this tests well 👍 Thank you for working on this improvement @afercia!

[afercia]

Thanks @jorgefilipecosta