Documentation: net: Document resilient next-hop groups

Add a document describing the principles behind resilient next-hop groups,
and some notes about how to configure and offload them.
Suggested-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/index.rst

@@ -76,6 +76,7 @@ Contents:
    netdevices
    netfilter-sysctl
    netif-msg
+   nexthop-group-resilient
    nf_conntrack-sysctl
    nf_flowtable
    openvswitch