doc: net: Rewrite network driver documentation

doc/README.drivers.eth seems like a good source for understanding
U-Boot's network subsystem, but is only talking about legacy network
drivers. This is particularly sad as proper documentation would help in
porting drivers over to the driver model.

Rewrite the document to describe network drivers in the new driver model
world. Most driver callbacks/methods are almost identical in their
semantic, but recv() differs in some important details.

Also keep some parts of the original text at the end, to help
understanding old drivers. Add some hints on how to port drivers over.

This also uses the opportunity to reformat the document in reST, on the
way moving it into doc/driver-model and adding it into the structure
there.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>