Create a convert-help.bash script to extract builtin documentation as
reStructuredText sources in a new Help directory.  Run each executable
with the --help-full option targeting a .rst file to extract the
documentation.  Generate Sphinx "toctree" directives to point each man
page at the corresponding documents it should contain.  Organize
cmake-commands(7), cmake-properties(7), and cmake-variables(7) man pages
into sections similar to those generated by --help-properties and
--help-variables output previously.