doc: add switch representation documentation

doc: add switch representation documentation

Add document to describe the  model for representing switching capable
devices in DPDK, using a general ethdev port model and through port
representors. This document also details the port model and the
rte_flow semantics required for flow programming, as well as listing
some example use cases.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Signed-off-by: Declan Doherty <declan.doherty@intel.com>
Reviewed-by: Marko Kovacevic <marko.kovacevic@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>