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: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>