I mean, since Doctor Who has no canon (aside from The Adventure Games and Infinity :>), by definition everything is extra-canon. The Star Wars canon system was created by Lucas Licensing. The BBC has done no such thing for Doctor Who, so any attempt to do so would not be correct, as canon is about authority. "Validity" simply refers to what sources are allowed to be used in the body of an article from an in universe perspective, so it would be wholly meaningless to adopt a stratified approach to validity.