Add mock graphic separators

To illustrate where we're putting graphic elements, add an
example of a courtroom sketch and gray separator bars.

I think the graphic elements look better if they take up the full
viewport width, so remove the padding from the body and instead
use the class column in the Tarbell spreadsheet to add the
Bootstrap ``container`` class.

Break the graphic elements into their own "slides" so they can
be contstrained within a container or take up the full width at
our discretion.
Merge branch 'master' into two-tier-nav

Integrate some of my layout changes with @bepetersn's nav
templating.

I couldn't get the num_slides macro to work.  The extension that
provides the ``do`` keyword wasn't loaded and I wasn't sure how
to do this within Tarbell.  It was easy enough to just iterate
through the slides and use an ``if`` statement.

Also update ``index.html`` to render nested sections for both
the top-level and slide elements.

Make the circles links to the slides.

Conflicts:
	css/simple.css
	slides/intro.md