Separate book deployment about site

[LeoDog896]

(This also means that the book submodule doesn't have to be constantly updated).

[nathanielknight]

This seem pretty sensible to me; the site and the book will probably have quite different release cadences in the long run.

That said, I'm not sure what the situation with the domain is, and getting book.pest.rs pointing at a separate GitHub Repo might not be possible (certainly not quickly).

Some options that occur to me are:

• putting the book on a GitHub Pages domain and re-directing from pest.rs/book (which we can get done in this repo)
• pulling the book into this repo as a sub-directory, which seems not ideal, but also it's how Rocket, Clap, Nom, and a bunch of other big crates seem to do it 🤷
• putting this on ice until the domain situation can move forward

[LeoDog896]

I think we should probably do the first option now, then, once the domain issue is (hopefully) resolved, then we can move forward with the second option.