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.