iommufd: Document overview of iommufd

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.com
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>