Utilities/Sphinx: Add role and directive for 'genex' in CMake domain

This enables cross-reference syntax for CMake generator expressions:

    :genex:`SOME_GENEX`
    :genex:`$<SOME_GENEX>`
    :genex:`$<SOME_GENEX:...>`

and definition of CMake generator expressions via a directive:

    .. genex:: SOME_GENEX
    .. genex:: $<SOME_GENEX>
    .. genex:: $<SOME_GENEX:...>

It also adds generator expressions defined by the directive and by
`Help/genex/SOME_GENEX.rst` documents to the index.

Source/cmRST.cxx

@@ -25,7 +25,7 @@ cmRST::cmRST(std::ostream& os, std::string docroot)
   , Markup(MarkupNone)
   , Directive(DirectiveNone)
   , CMakeDirective("^.. (cmake:)?("
-                   "command|envvar|variable"
+                   "command|envvar|genex|variable"
                    ")::[ \t]+([^ \t\n]+)$")
   , CMakeModuleDirective("^.. cmake-module::[ \t]+([^ \t\n]+)$")
   , ParsedLiteralDirective("^.. parsed-literal::[ \t]*(.*)$")
@@ -37,7 +37,8 @@ cmRST::cmRST(std::ostream& os, std::string docroot)
   , NoteDirective("^.. note::[ \t]*(.*)$")
   , ModuleRST(R"(^#\[(=*)\[\.rst:$)")
   , CMakeRole("(:cmake)?:("
-              "command|cpack_gen|generator|variable|envvar|module|policy|"
+              "command|cpack_gen|generator|genex|"
+              "variable|envvar|module|policy|"
               "prop_cache|prop_dir|prop_gbl|prop_inst|prop_sf|"
               "prop_test|prop_tgt|"
               "manual"