Documentation/sphinx: add Sphinx kernel-doc directive extension

Add an extension to handle kernel-doc directives, to call kernel-doc
according to the arguments and parameters given to the reStructuredText
directive.

The syntax for the kernel-doc directive is:

.. kernel-doc:: FILENAME
   :export:
   :internal:
   :functions: FUNCTION [FUNCTION ...]
   :doc: SECTION TITLE

Of the directive options export, internal, functions, and doc, currently
only one option may be given at a time.

The FILENAME is relative from the kernel source tree root.

The extension notifies Sphinx about the document dependency on FILENAME,
causing the document to be rebuilt when the file has been changed.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>