Headings for examples

[mattgarrish]

Is your feature request related to a problem? Please describe.
We've had a request to change the examples so they have proper headings so screen reader users can navigate to them more easily.

Describe the solution you'd like
Instead of using a div with class marker on the example headings, add role=heading + aria-level like is done for note headings.

Describe any alternatives you've considered
I'm considering a post-processing step to find all the divs and convert them to headings, but if this is generally useful it seems like it would be helpful if it were built into all specs.

I can't find anything in the issues about why examples are treated differently from notes, so if there's a problem that makes this untenable I'd love to hear about it before I go and hack at the problem.