Separate format1 and format2 documentation.

Fixes ros/catkin#440.

 * The main how-to index is for format2. It refers format1 users to a subdirectory.

 * The format2 version recommends `<depend>`, but does not yet mention `<devel_depend>`
or `<exec_depend>`.

 * The format1 version does not mention `<depend>`.