A thing we could have is an introduction section. A place on the website for new users to get a feel for the program. We could have little tutorials, and links to places for extra help, such as the Forums. And maybe an advanced section, for people who want more out of the experience, that would show some more complicated features.
I could write it too, if I have permission from the higher-ups that is.
You don't need permission; you could make a wiki post on the forum. I think we'd want to see the result before giving it any sort of official blessing. But I agree, we need something like that, so go for it!
I think your first big, overarching decision to make is whether or not to assume your reader has already programmed in Scratch.
Sure. But I was thinking that the beginning part would take a lot more effort, because you can't just throw around words like "palette," etc. It has to be done super carefully. It has to explain turtle vs. Euclidean geometry, for example. So you might never make it to the (in my mind) interesting parts. This is why the Reference Manual assumes that you know Scratch.
I agree with the manual assuming the user know scratch, since it's sometimes annoying to find a tutorial for something, only for it to explain the very basics, even though you already know it. I think that if someone were to use snap, they'd at least know a little bit about scratch.