Added documentation draft
This merges in the first draft of the Sphinx-based ModelBuilder User Manual.
@dcthomp The build currently uses Makefiles and should be changed to integrate into CMB's CMake build system. A webhook with CMB to readthedocs.org should probably be set up as well.
Sorry for all the closed merge requests; Gitlab removed the ability to change source branches (it is reimplemented in gitlab recently...). The buildbot didn't like the make.bat file even if I delete it (since it is in the commit history).