Add Doxygen documentation for SlicerRT source code

[cpinter]

Using doxygen will create a need for us to better document the C++ code. Plus we can throw in some extra user manual info into the html.

Plan on how to proceed:

first step is to manually generate the html and push to a website somewhere and have it link to Assembla. need to ask Andras about where to host it.

Next step to automatically generate the html.

at last is to add other user manual stuff to doxygen.

Migrated from https://app.assembla.com/spaces/slicerrt/tickets/709-add-doxygen-documentation-for-slicerrt-source-code/details