Add an ‘article’ layout and factor out byline CSS

[dempseyatgithub]

We added the first stand-alone documentation article to swift.org.

The contribution byline is at the bottom of the article, but has the same style as bylines in the heading of blog posts.

This is currently in the one existing article ‘Value and Reference Types in Swift’.

This common footer should be factored out into a separate ‘article’ layout. The styles for the byline should also likely be factored into classes that don't depend on the HTML tag containment hierarchy.