docs: net: add an explanation of VF (and other) Representors

There's no clear explanation of what VF Representors are for, their
 semantics, etc., outside of vendor docs and random conference slides.
Add a document explaining Representors and defining what drivers that
 implement them are expected to do.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20220905135557.39233-1-ecree@xilinx.com


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