New 3D tutorial using POV-Ray

This describes the current state of 3D diagrams --- what will be 1.1.  If we're hinting in the release notes that 3D diagrams are possible, I think it would be nice to get this or something like it on the website.  

Maybe it's also time to put `diagrams-povray` on Hackage?  I'm not sure; I've definitely been leaning towards "No documentation yet, not ready for public consumption."