move scripts to separate include so we can include them across layouts even if the footer isn't there

Additional markup for the skip link (to ensure it has the same effective
width as the main content, and to provide an outline around just the
link text) plus some basic styles...make it look a bit more in keeping
with the overall look and feel of the docs.

http://blog.paciellogroup.com/2013/02/using-wai-aria-landmarks-2013/#tablex

http://www.w3.org/TR/wai-aria/roles#contentinfo

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.

* 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)