Utilities/Sphinx: Index guide-level documents for cross-referencing

Extend the change from commit d2fde94809 (Help: Add infrastructure for
guide-level documentation, 2019-05-30, v3.16.0-rc1~531^2~4) to add
support for cross-referencing and indexing the guides.

Utilities/Sphinx/cmake.py

@@ -191,6 +191,7 @@ _cmake_index_objs = {
     'cpack_gen':  _cmake_index_entry('cpack generator'),
     'envvar':     _cmake_index_entry('envvar'),
     'generator':  _cmake_index_entry('generator'),
+    'guide':      _cmake_index_entry('guide'),
     'manual':     _cmake_index_entry('manual'),
     'module':     _cmake_index_entry('module'),
     'policy':     _cmake_index_entry('policy'),
@@ -373,6 +374,7 @@ class CMakeDomain(Domain):
         'cpack_gen':  ObjType('cpack_gen',  'cpack_gen'),
         'envvar':     ObjType('envvar',     'envvar'),
         'generator':  ObjType('generator',  'generator'),
+        'guide':      ObjType('guide',      'guide'),
         'variable':   ObjType('variable',   'variable'),
         'module':     ObjType('module',     'module'),
         'policy':     ObjType('policy',     'policy'),
@@ -407,6 +409,7 @@ class CMakeDomain(Domain):
         'cpack_gen':  CMakeXRefRole(),
         'envvar':     CMakeXRefRole(),
         'generator':  CMakeXRefRole(),
+        'guide':      CMakeXRefRole(),
         'variable':   CMakeXRefRole(),
         'module':     CMakeXRefRole(),
         'policy':     CMakeXRefRole(),

Utilities/Sphinx/create_identifiers.py

@@ -25,6 +25,7 @@ for line in lines:
              ("envvar", "envvar"),
              ("variable", "variable"),
              ("generator", "generator"),
+             ("guide", "guide"),
              ("target property", "prop_tgt"),
              ("test property", "prop_test"),
              ("source file property", "prop_sf"),