This allows to build PDF of only the sub-projects, which reduce the
roundtrip compared to build the PDF from the main documentation.
Signed-off-by: NMarkus Heiser <markus.heiser@darmarit.de>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

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>

With the gpu/conf.py, the gpu folder can be build and distributed
stand-alone. To compile only the html of 'gpu' folder use::

  make SPHINXDIRS="gpu" htmldocs
Signed-off-by: NMarkus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>