Help: Restore installation of top-level index

We removed installation of `Help/index.rst` in commit d2fde94809 (Help:
Add infrastructure for guide-level documentation, 2019-05-30), but the
file is required for `--help-full` to work.  Restore installation of the
file and update it to avoid referencing the `Help/guide` directory in
its toctree during processing by `cmRST`.

CMakeLists.txt

@@ -813,7 +813,7 @@ if(NOT CMake_TEST_EXTERNAL_CMAKE)
     PATTERN "*.sh*" PERMISSIONS OWNER_READ OWNER_EXECUTE OWNER_WRITE
                                 GROUP_READ GROUP_EXECUTE
                                 WORLD_READ WORLD_EXECUTE
-    REGEX "Help/(dev|guide|index.rst)($|/)" EXCLUDE
+    REGEX "Help/(dev|guide)($|/)" EXCLUDE
     )
 
   # Install auxiliary files integrating with other tools.

Help/index.rst

@@ -44,13 +44,15 @@ Reference Manuals
    /manual/cmake-variables.7
    /manual/cpack-generators.7
 
-Guides
-######
+.. only:: not man
 
-.. toctree::
-   :maxdepth: 1
+ Guides
+ ######
+
+ .. toctree::
+    :maxdepth: 1
 
-   /guide/tutorial/index
+    /guide/tutorial/index
 
 .. only:: html or text