ice: Add documentation for devlink-rate implementation

Add documentation to a newly added devlink-rate feature. Provide some
examples on how to use the commands, which netlink attributes are
supported and descriptions of the attributes.

Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>