They belong to you...not the car. OK, it’s a part of the cars history but you don’t have to let the new owner have the documents if you want to retain them for old times sake.
I put all the paperwork together for Nero but retained the purchase invoice....I keep them for all my cars. If the new owner contacts me I might let him/her have a copy just as I might let them have some photos or a copy of the Caramulo photo book.