[ #2386] Add hooks-ddns-tuning to exclude duplicate in conf.rst
Quick fix to repair duplicate warning in documentation building
/home/docs/checkouts/readthedocs.org/user_builds/kea/checkouts/latest/doc/sphinx/arm/hooks-ddns-tuning.rst:4: WARNING: duplicate label hooks-ddns-tuning, other instance in /home/docs/checkouts/readthedocs.org/user_builds/kea/checkouts/latest/doc/sphinx/arm/hooks.rst