Azkaban documentation redesign

This fixes #114.

 * Apply new Azkaban Jekyll theme.
 * 2.1 and 2.5 documentation now use single-page layout.
 * In order to support single-page layout without installing additional Jekyll plugins (since GitHub pages disallows additional Jekyll plugins):
 ** documents renamed as docs
 ** docs/2.1 and docs/2.5 only contain a single index.html, which includes each of the sub-pages that make up the individual documentation sections.
 ** All documentation sub-pages converted from Markdown to HTML and are now located in _includes/docs/2.x