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

Documentation/networking/index.rst

@@ -92,6 +92,7 @@ Contents:
    radiotap-headers
    rds
    regulatory
+   representors
    rxrpc
    sctp
    secid

Documentation/networking/switchdev.rst

 .. SPDX-License-Identifier: GPL-2.0
 .. include:: <isonum.txt>
+.. _switchdev:
 
 ===============================================
 Ethernet switch device driver model (switchdev)