Install the Help directory next to Modules to make it available in CMake
distributions.  Use cmRST to read Help .rst documents and print them as
help output.

Add options

 --help-manual-list
 --help-manual

to list available manuals or print one of them.  Implement the options

 --help-commands
 --help-modules
 --help-policies
 --help-properties
 --help-variables

by mapping to the corresponding manual page.  Implement the options

 --help-command-list
 --help-module-list
 --help-policy-list
 --help-property-list
 --help-variable-list

by globbing the available Help .rst documents of the corresponding type
and reading their titles.  Implement the options

 --help-command
 --help-module
 --help-policy
 --help-property
 --help-variable

by globbing the matching Help .rst document(s) and printing them.