Bug: Popover examples with no focusable content do not move focus to popover surface

[smhigley]

We should expose the content of the popover in some way to screen reader users. If a popover were functioning as a tooltip, we could use something like aria-describedby without moving focus, but for all the examples in the docs page, it looks like we should be moving focus to the PopoverSurface, which we can do by adding tabindex="-1".

Creating an issue for myself to do this, and add authoring docs.