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

Commit 08f588fa301bef ("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/[e-mail]/
Fixes: 08f588fa301bef ("devlink: introduce framework for selftests")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Bagas Sanjaya <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/devlink/index.rst

···
    devlink-region
    devlink-resource
    devlink-reload
+   devlink-selftests
    devlink-trap
    devlink-linecard