doc/sphinx: Stop touching state_machine internals

Instead of just forcefully inserting our kernel-doc input and letting
the state machine stumble over it the recommended way is to create
ViewList, parse that and then return the list of parsed nodes.

Suggested by Jani.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: linux-doc@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>