doc-rst: add index to sub-folders

Add a index if only a sub-folder is build e.g.::

  make SPHINXDIRS=media cleandocs htmldocs

BTW: removed dead search link in the top-index file
Signed-off-by: NMarkus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/gpu/conf.py

 # -*- coding: utf-8; mode: python -*-
 
 project = "Linux GPU Driver Developer's Guide"
+
+tags.add("subproject")

Documentation/gpu/index.rst

@@ -12,3 +12,10 @@ Linux GPU Driver Developer's Guide
    drm-uapi
    i915
    vga-switcheroo
+
+.. only::  subproject
+
+   Indices
+   =======
+
+   * :ref:`genindex`

Documentation/index.rst

@@ -19,4 +19,3 @@ Indices and tables
 ==================
 
 * :ref:`genindex`
-* :ref:`search`

Documentation/media/conf.py

 # -*- coding: utf-8; mode: python -*-
 
 project = 'Linux Media Subsystem Documentation'
+
+tags.add("subproject")

Documentation/media/index.rst

@@ -10,3 +10,10 @@ Contents:
    media_kapi
    dvb-drivers/index
    v4l-drivers/index
+
+.. only::  subproject
+
+   Indices
+   =======
+
+   * :ref:`genindex`