Add doc about behavior of `typst-book init`

[Myriad-Dreamin]

It is expected to ask a few questions, generate a book project, and then serve the book locally for you.

I was copying the description "It will ask a few questions before generating the book...", which is more friendly, but I have not implement the prompt for asking some questions yet. As for starting a static server after generating the book, It should have worked as expected.

That's what I mean.