Along the way, our flat structure for docs was changed
to categorize the documents, but service_levels.md was forward-ported
later and missed the created directory structure, so it was created
as a sole document in the top directory. Move it to where the other
similar docs live.
Message-Id: <68079d9dd511574ee32fce15fec541ca75fca1e2.1640248754.git.sarna@scylladb.com>
