Document Markdown support.
This change adds some relatively thorough documentation for using markdown.

The bulk of this is a page called "Using Markdown" which gives a quick overview

of the basics of markdown syntax for the things most people will care about. I

haven't made this an exhaustive document, since the pages on daring fireball

and github serve that purpose. Instead, this should hopefully be a nice quick

reference for the basics of markdown syntax as interpreted by Review Board.
This is linked to from all the various places where markdown can be used.
While I was in here, I updated everything to capitalize "Markdown", including

in the MD review UI docs, and capitalized the terms in the glossary (which

looks much better to me).

Generated HTML docs and read through everything.
Checked cross-linking.