First pass at building documentation.

Both Doxygen (ninja/make target `doc`)
and Sphinx (ninja/make target `doc-userguide`)
documentation are built.
The Doxygen documentation is put in `documentation/reference/SimMedTK`
while the Sphinx documentation is put in `documentation/user`.

This should close issue #3. (Actually finishing writing the
documentation should probably be several separate issues covering more
specific parts of the documentation.)