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

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: 72ba11079cbe ("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.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>