Add iommufd into the documentation tree, and supply initial documentation.
Much of this is linked from code comments by kdoc.

Link: https://lore.kernel.org/r/5-v6-a196d26f289e+11787-iommufd_jgg@nvidia.comReviewed-by: NBagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: NEric Auger <eric.auger@redhat.com>
Signed-off-by: NKevin Tian <kevin.tian@intel.com>
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>