Reorganization of developer documentation

This pull request is not intended to be a complete overhaul of the developer docs; it is a rough outline of changes to the front page and an example of what is intended to be a very new-to-git friendly intro to contributing.

Please speak up if you need to see more detail before making comments. I can pretty quickly outline what the rest might look like.

@eteq @embray @demitri were interested in this on astropy-dev; I understand there might not be feedback until 0.3 is out.