docs: netdev: document requirements for Supported status

As announced back in April, require running upstream tests
to maintain Supported status for NIC drivers:

  https://lore.kernel.org/20240425114200.3effe773@kernel.org



Multiple vendors have been "working on it" for months.
Let's make it official.

Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250111024359.3678956-1-kuba@kernel.org


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