Documentation: devlink: add add devlink-selftests to the table of contents

Commit 08f588fa ("devlink: introduce framework for selftests") adds
documentation for devlink selftests framework, but it is missing from
table of contents.

Add it.

Link: https://lore.kernel.org/linux-doc/202207300406.CUBuyN5i-lkp@intel.com/
Fixes: 08f588fa ("devlink: introduce framework for selftests")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20220730022058.16813-1-bagasdotme@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/devlink/index.rst

@@ -38,6 +38,7 @@ general.
    devlink-region
    devlink-resource
    devlink-reload
+   devlink-selftests
    devlink-trap
    devlink-linecard