Clean up dev doc organization, especially sphinx extensions

This started as a clean-up of the sphinx extension documentation, as discussed in #844 . It does that but has also evolved into a rather more significant re-arrangement of the developer section to (I think) make the structure more clear and generally make individual pages less intimidating.

You can see the build result at http://eteq.github.com/astropy/#developer-documentation

@mdboom @astrofrog @iguananaut, as it deals mostly with doc sections you've had significant hands in.