This PR significantly improves Bootstrap's accessibility for users of assistive technology, such as screen readers. Some of the these changes add additional markup to the source examples, but we believe that the sacrifice in readability is worth achieving more widespread usage of accessibility best-practices.

What was done
- Added lots of [WAI-ARIA attributes](http://www.w3.org/WAI/intro/aria)
- Added `.sr-only` helper class, that is only readable by screen readers (and invisible for all other users). This lets us - make progress bars and paginations accessible to screen reading users.
- Advised users to always use label elements. For inline forms, they can hide them with `.sr-only`
- Added 'Skip navigation' link
- Added "Accessibility" section to getting-started.html.

What *wasn't* done
- Contrast issues (twbs#3572)
- Tooltips (twbs#8469)
- Documentation re: usage of icons, since they now live in a separate repo

Major props to all that contributed: @bensheldon, @jasonlally, @criscristina, and @louh. Feel free to chime in, guys, if I've left anything out.

/cc @cv

In class names:
* large => lg
* small => sm
* mini => xs ("Extra small")

In screen size categories:
* Tiny => Extra small

While examples are phenomenally useful and awesome for folks getting
started with Bootstrap, they don't really shine as well being buried in
the main docs. Moreover, having anything that's not core HTML, CSS, JS,
or documentation in the main repo seems to just get in the way.
Therefore, examples gets its own repo.

Specifically, we're doing the following:

* Deletes all example screenshots
* Deletes example layout
* Deletes example files
* Updates Getting Started docs to include only the template and a link
to the new examples project

All the examples and their history will be preserved in the switch.

* Bring back the navbar up top
* Pull the docs content back into separate pages because a 6.6k line docs file is just crazy
* Recenter the page content and bring back affixed side nav (needs work)