Add Protected paths topic to global navigation

While looking at: gitlab#330225 (closed), noticed the relevant documentation wasn't on the global nav:

• Before: https://docs.gitlab.com/ee/user/admin_area/settings/protected_paths.html#protected-paths
• After: http://eread-add-protected-paths-topic-to-nav.178.62.207.141.nip.io/ee/user/admin_area/settings/protected_paths.html#protected-paths