Get things going on readthedocs.org.
This fixes semi-infinite recursive copy caused by an incorrect directive in the Sphinx configuration file; the directory for static HTML content should be specified or omitted, but not passed an invalid dummy value.
Also,
- add some high-level documentation,
- rearrange top-level structure to make separation between CMB and ModelBuilder clear, and
- add a Python virtualenv requirements file specifying dependent packages required to build the documentation.