A link with "?" most likely means that the technology, era etc. Does not exist, is unknown or varies. Usually "?" is used if the unit, building, wonder etc. has not been released and will be in an upcoming expansion, game or DLC.
Sometimes articles may have a "?" but the technology/era etc. is available - this is probably due to lack of edits (i.e. at the time it was not available) or human error (they did not know). It is appreciated that you change this if you know the era/technology etc.