devlink: Add Documentation/networking/devlink-health.txt

This patch adds a new file to add information about devlink health
mechanism.

Signed-off-by: Aya Levin <ayal@mellanox.com>
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>