Onboarding improvements

[cellio]

We have some good guidance for new users, but we can do better. In reviewing qpixel through the hypothetical eyes of a new user, I had the following ideas:

• Our signup page is pretty no-nonsense/utilitarian. Maybe we could add some welcome language there? This is a first-impressions moment, so even if it's not necessary, it could be helpful.
• Confirmation email has a link to complete registration. We know what community the user signed up on, so maybe also link the local FAQ in the help center? Or maybe the tour? (Yes there will be a notification, one of two or three that will be waiting for the user.)
• If a community is in "new site" mode, let's suppress the notification for the Participate ability, which is immediately superseded by Participate Everywhere. If we can't suppress it, let's at least send them in order, so Participate Everywhere is at the top. Otherwise, the user reading down the inbox sees: stuff I can't do, oh ok stuff I can do, and then the tour.
• We get the question "how are you different from Stack Overflow?" fairly often. We have some meta Q&A about that, and information about our platform is in the help and the UI in various places, but can we be more proactive? Can we put "How is Codidact different from other platforms" front and center for new users somewhere? Maybe we should have a help topic that is specifically an introduction for new users, and on our network that would include distinctions between us and SO (and Reddit?). We could then link that topic somewhere that new users will see -- inbox notification, tour, dismissible widget, something else?

We also have a broad goal of providing more just-in-time guidance, but I'm focusing here on the initial onboarding.