[✨] improve 404 pages

[naorpeled]

At the moment when you reach a 404 page in a QwikCity app, you only see a 404 message.

What I'm proposing is that in addition to the message, you'll also see all the available routes, like what Gatsby does (here).

@manucorporat this is something we've discussed a while ago but didn't have the time to implement it. Let me know if this is greenlit, if so I'd like to take this task now.