Documentation: net: move device drivers docs to a submenu

Some of the device drivers have really long document titles
making the networking table of contents hard to look through.
Place vendor drivers under a submenu.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Dave Watson <davejwatson@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>