docs: networking: Add failover docs to index

Currently we have rst format docs for the failover and net_failover
modules however these docs are not linked to within the index.

Add `failover` and `net_failover` to the networking documentation index.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>