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

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

Commit 67c093b82b7b83 ("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: 67c093b82b7b83 ("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.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>