Implement minimal Doxygen creation

Add doc subdirectory for holding Doxygen configuration and as a
placeholder for future items. Add CMake option, on by default, to
build Doxygen docs.

Provide minimal Doxy file, using CMake to configure the INPUT_PATH
only at present. Add custom_command/target pair to run Doxygen
over the configuration as part of the ALL target. Install generated
html documentation to CMAKE_INSTALL_DOCDIR.