devlink: introduce devlink-dpipe.rst documentation file

Primarily based on the DPIPE netdev conference paper, introduce a new
file to document the dpipe interface.

This likely needs further improvement, but is at least a good overall
start.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>