Rebase typography off of mdn

Re-based the template typography off of the Mozilla Developer Network site so generated API docs have a more contemporary and familiar style. In addition to CSS changes, two new headings were added ('Summary' and 'Constructor') to visually improve sections of the page that previously had no heading. This _pull-request_ is marginally dependent on #741 since one of added headings is in a part of the template that previously never existed.