Use `aria-current` where appropriate

Created by: ffoodd

aria-current attribute has many advantages and a single drawback compared to visually hidden text like <span class="sr-only">(current)</span> — let me explain:

• it's standardized as part of ARIA 1.1
• it's widely supported by browsers and screen readers
• it's interoperable and programmatic, so can be usedby any tool reading the markup
• internazionalition and localization is not a thing, assistive technologies take care of it
• it allows to fine tune the context with its diffrent values (page, true, date, step, location, etc.)
• it could be used as a style hook by using [aria-current], thus replace the .active state (didn't push this much, but could if you like the idea) — and also as a JavaScript hook, obviously.

The single drawback is that is does not appear when CSS is missing, whereas the visually hidden text does.

I think aria-current wins, so here's a full PR making a few different things:

• add aria-current in components already using .active (only if appropriate, obviously — so not in JavaScript tabbed interface, for example) ;
• mention aria-current beside .active in thos components' documentation;
• update examples templates accordingly;
• apply aria-current in docs navigations as well (main nav, sidenav, versions dropdowns).

And as usually, each of those lives in its very own commit.