Merge branch 'conf_groups' into 'master'

doc/man/knot.conf.5in

doc/reference.rst

src/knot/conf/conf.c

src/knot/conf/conf.h

src/knot/conf/schema.c

src/knot/conf/schema.h

src/knot/conf/tools.c

src/knot/conf/tools.h

src/knot/dnssec/context.c

src/knot/events/handlers/ds_push.c

src/knot/events/handlers/notify.c

src/knot/updates/acl.c

src/knot/zone/zone.c

src/libknot/yparser/ypschema.c

src/libknot/yparser/ypschema.h

tests/knot/test_conf.c