Help: Add new section for CPack generators

The documentation for CPack generators previously lived in their
respective internal CMake modules. This setup was misleading,
because it implied that you should include the modules in your own
code, which is not the case. Moving the documentation into a
separate section does a better job of hiding the internal modules,
which are just an implementation detail. The generator documentation
has also been modified to remove any references to the module name.

The CPackIFW module is a special exception: since it has user-facing
macros, the documentation for these macros has been kept in the module
page, while all other documentation related to the IFW generator has
been moved into the new section.

To make it easier to find the new documentation, the old help pages
for the CPack*.cmake modules have not been deleted, but have been
replaced with a link to their respective help page in the new
documentation section.

Tests/CMakeLib/testRST.expect

@@ -19,6 +19,7 @@ Variable ``VARIABLE_<PLACEHOLDER>`` with trailing placeholder and target.
 Environment variable ``SOME_ENV_VAR``.
 Environment variable ``some env var`` with space and target.
 Generator ``Some Generator`` with space.
+Generator ``Some Generator`` with space.
 Inline literal ``~!@#$%^&*( )_+-=\\[]{}'":;,<>.?/``.
 Inline link Link Text.
 Inline link Link Text <With \-escaped Brackets>.

Tests/CMakeLib/testRST.rst

@@ -26,6 +26,7 @@ Variable :variable:`VARIABLE_<PLACEHOLDER> <target>` with trailing placeholder a
 Environment variable :envvar:`SOME_ENV_VAR`.
 Environment variable :envvar:`some env var <SOME_ENV_VAR>` with space and target.
 Generator :generator:`Some Generator` with space.
+Generator :cpack_gen:`Some Generator` with space.
 Inline literal ``~!@#$%^&*( )_+-=\\[]{}'":;,<>.?/``.
 Inline link `Link Text <ExternalDest>`_.
 Inline link `Link Text \<With \\-escaped Brackets\> <ExternalDest>`_.