Adding detailed documentation of the markdown syntax.

Many new users of a GitLab instance want to know the markdown syntax as well as its rendered result.  Rather than referencing the Daring Fireball link, which is verbose and has no rendered output, here, the markdown syntax is explained directly in the help document with rendered examples.  Also, by moving the markdown help to markdown itself, it allows users to see exactly how the page is formatted.  This greatly aids in helping GitLab users learn the markdown they plan to use and an easy place for reference.