fwctl: Add documentation

Document the purpose and rules for the fwctl subsystem.

Link in kdocs to the doc tree.

Link: https://patch.msgid.link/r/6-v5-642aa0c94070+4447f-fwctl_jgg@nvidia.com


Nacked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20240603114250.5325279c@kernel.org


Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/ZrHY2Bds7oF7KRGz@phenom.ffwll.local


Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Shannon Nelson <shannon.nelson@amd.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>