Add breakpoint for content section of docs

Enables a mobile device to read the documentation without scrolling back
and forth for each line.

Fix: < 767px, display content as 100% of the viewport with stacked
span elements. Previously, they were fixed-width elements that must be
clipped.

 - Borrowed CSS from bootstrap-responsive, but restricted it to the
   #content div. Dropping it in raw made a mess of the footer.

 - + fixed-width to header/navs to maintain their integrity (althought
   they remain non-responsive)

Signed-off-by: Will O'Brien <will.obrien@dibit.co>