Documentation: networking: Add description for multi-pf netdev

Add documentation for the multi-pf netdev feature.
Describe the mlx5 implementation and design decisions.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>