Add aria-label or aria-describedby attributes to all <nav> elements

Coming from #2045039-7: Cleanup HTML heading structure @terrill:

That said, I still think we should add aria-label attributes to all elements. There are many of these on each page, and without a label they're each identified by screen readers as "Navigation region" - there's no means of clarifying which navigation region is which. If we add aria-label, screen readers use the value of aria-label when identifying the region (e.g., "Toolbar Items Navigation Region", "Breadcrumb Navigation Region", and the like)

@falcon03 and later on:

Well, I think that hearing messages like "entering navigation", "entering main" and so on doesn't give a real "context". So, as a blind user, I'm completely in favor of adding aria-labels.

@pratikp1 and finally:

Having an effective label is essential. I would like some discussion about that for D8 or D9. Let's see if we can make it more understandable. It will go a long way toward having users understand that landmarks can actually be useful.

I don't know why there isn't any in Seven, but in Bartik, we can find the <nav> element in:
core/themes/bartik/templates/comment.html.twig
core/themes/bartik/templates/page.html.twig

And in modules it's in:
core/modules/book/templates/book-all-books-block.html.twig
core/modules/book/templates/book-navigation.html.twig
core/modules/system/templates/breadcrumb.html.twig
core/modules/system/templates/page.html.twig
core/modules/toolbar/toolbar.module

Where else do we find them?

Some more info on using aria-labels:
http://www.last-child.com/debugging-aria-label/