Help: Add roles to abbreviate command-line options

Add a handful of helper roles that allow many more command-line option
references to be written without explicit targets. Specifically, what
used to be written ':option:`--foo[=bar] <program --foo>` can now be
written ':program-option:`--foo`'. This allows many such references to
be significantly shortened.

For now, supported programs are 'cmake', 'cpack'. 'ctest',
'cmake--build' and 'cmake--install', which should cover most cases, but
more can be trivially added. Additionally, this does not make a
concerted effort to replace all existing uses, as it is intended mainly
for use in future updates.