doc: networking: add offload documents into main index file

This patch just adds references to offload documents into main table of
contents in network documentation.
Signed-off-by: NOtto Sabart <ottosabart@seberm.com>
Acked-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/networking/index.rst

@@ -32,6 +32,8 @@ Contents:
    alias
    bridge
    snmp_counter
+   checksum-offloads
+   segmentation-offloads
 
 .. only::  subproject