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

Documentation/networking/index.rst

@@ -15,6 +15,8 @@ Contents:
    kapi
    z8530book
    msg_zerocopy
+   failover
+   net_failover
 
 .. only::  subproject