standardize orientation docs across models

[pkienzle]

All oriented shape models should point to the orientation docs rather than repeating the information in each model. You should be able to link to it using :ref:`orientation`.

It would be helpful to talk about orientations with regard to the 1-2, 1-3 and 2-3 planes, and with regard to radial and transverse measurements in the shear cell. That is, indicate what values of theta, phi and psi should be used assuming the long axis of the particle is oriented with the flow.