You can do that with any online dictionary, however. The only thing different about that one is it has no additional features. This is your one stop word-lookup link
Yes, it is acting as the question indicator particle. I could omit it but if I were to omit in speech then I have to make sure to keep the rising intonation to indicate that I'm asking a question. Otherwise it would be construed as a statement. Of course in writing you have to have something to indicate a question though.
だれがこのウエブサイトを作ったの sounds more natural. だれがこのウエブサイトを作ったのですか as well. The sentence is read as an interrogative one, since the interrogative だれが is at the front.
more simply, just add "?" ;-)