Allow OpenSim doxygen to link to Simbody doxygen.

When browsing your OpenSim doxygen, have you ever come upon a class, say
SimTK::MobilizedBody, and wanted to see the documentation for it? Well, now,
OpenSim doxygen will contain a link right there, as if the OpenSim and Simbody
doxygen were combined. Check out the result:
http://stanford.edu/~dembia/opensim/classOpenSim_1_1Body.html.

On your local machine, the wiring is done automatically (presuming you've built
Simbody doxygen locally). If posting the Doxygen online, one must set the
OpenSim CMake variable `OPENSIM_SIMBODY_DOXYGEN_LOCATION` to a valid URL where
Simbody doxygen exists. That's what I've done for the above URL.

This is made possible by https://github.com/simbody/simbody/pull/203.

Addresses some of #122. Our doxygen homepage still contains HTML with
hard-coded URL's to Simbody's doxygen. I don't know how to fix that, but I
think this is a first step.

This is ready to go in.

EDIT: Don't click SimTK::Vec3; that link isn't working because the version of Simbody doxygen online is older than the one I used to generate OpenSim doxygen.
`Simbody_DOXYGEN_DIR` is given to us when we `find_package(Simbody)`