Documentation: Add documentation for eswitch attribute

Provide devlink documentation for three eswitch attributes:
mode, inline-mode, and encap-mode.

Signed-off-by: William Tu <witu@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20240325181228.6244-1-witu@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>