doc-rst: include index only in subproject AND html

The index should only be included if the build of the sub-folder is done
with the html-builder (HTML output).
Signed-off-by: NMarkus Heiser <markus.heiser@darmarit.de>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/80211/index.rst

@@ -9,7 +9,7 @@ Linux 802.11 Driver Developer's Guide
    mac80211
    mac80211-advanced
 
-.. only::  subproject
+.. only::  subproject and html
 
    Indices
    =======

Documentation/admin-guide/index.rst

@@ -32,3 +32,11 @@ Contents:
    java
    bad-memory
    basic-profiling
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`
+

Documentation/gpu/index.rst

@@ -14,7 +14,7 @@ Linux GPU Driver Developer's Guide
    vga-switcheroo
    vgaarbiter
 
-.. only::  subproject
+.. only::  subproject and html
 
    Indices
    =======

Documentation/process/index.rst

@@ -30,3 +30,11 @@ Contents:
    volatile-considered-harmful
 
    development-process
+
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`