Build Help documentation during CMake build using Sphinx

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, generator, manual,
  module, policy, prop_*, and variable.  Add a "role" for each type
  to perform cross-references.  Teach the roles to treat "<XYZ>"
  as placeholders instead of explicit targets if not preceded by
  a space.  Add cmake domain directives to define command and
  variable objects explicitly in .rst file content.  This will
  allow modules to define their own commands and variables and
  have them indexed and linkable.

* A Sphinx document transform that converts Help/<type>/*.rst documents
  into cmake domain objects of the corresponding <type> and adds index
  entries for them.  This will automatically index all CMake documentation
  objects and provide cross-reference targets for them with no special
  markup in the .rst files.