net/smc: fix document build WARNING from smc-sysctl.rst

Stephen reported the following warning messages from smc-sysctl.rst

Documentation/networking/smc-sysctl.rst:3: WARNING: Title overline
too short.
Documentation/networking/smc-sysctl.rst: WARNING: document isn't
included in any toctree

Fix the title overline and add smc-sysctl entry into
Documentation/networking/index.rst

Fixes: 12bbb0d1 ("net/smc: add sysctl for autocorking")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Dust Li <dust.li@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220303113527.62047-1-dust.li@linux.alibaba.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/index.rst

@@ -96,6 +96,7 @@ Contents:
    sctp
    secid
    seg6-sysctl
+   smc-sysctl
    statistics
    strparser
    switchdev

Documentation/networking/smc-sysctl.rst

 .. SPDX-License-Identifier: GPL-2.0
 
-=========
+==========
 SMC Sysctl
-=========
+==========
 
 /proc/sys/net/smc/* Variables
-==============================
+=============================
 
 autocorking_size - INTEGER
 	Setting SMC auto corking size: