Add a Utilities/Sphinx directory to hold CMake build code to run the
Sphinx (sphinx-doc.org) documentation generation tool.  Create a
CMakeLists.txt file there capable of building either as a subdirectory
of the main CMake build, or as a standalone documentation build.

Add cache options SPHINX_MAN and SPHINX_HTML to select output formats
and SPHINX_EXECUTABLE to specify the sphinx-build executable.  Add
bootstrap options --sphix-man and --sphinx-html to select output formats
and --sphinx-build=<sb> to specify the sphinx-build executable.

Create a "conf.py.in" file to configure_file into "conf.py" to tell
sphinx-build how to build our documents.  Create a "cmake.py" Sphinx
extension module defining:

* The "cmake-module" directive used in Help/module/*.rst files to
  scan .rst markup from the corresponding Modules/*.cmake file.

* A Sphinx domain called "cmake" defining documentation object types
  for CMake Help/<type> directories: command, genera...