Rustup: `rustup doc --rustup` command to open the rustup book

[ickk]

Describe the problem you are trying to solve
rustup doc is capable of opening all kinds of rust books - alloc, cargo, embedded, nomicon, proc_macro, etc - however rustup can't open its own book.

Describe the solution you'd like
rustup doc --rustup should open the rustup book.

Notes
This would at least partially address online documentation being incoherent with the release a user has installed, or indeed incoherent with any currently available release #2713.

[kinnison]

Currently the _rustup book_ is not actually distributed as part of the documentation component because it's not one of the components of a Rust installation per-se. I suppose we could have it launch the http link, but I'm not sure that's entirely helpful since rustup doc is supposed to be offline documentation.

[jyn514]

Currently the rustup book is not actually distributed as part of the documentation component because it's not one of the components of a Rust installation per-se.

Does it make sense to include it with the distributed docs? I guess that gets tricky since not all distributions are installed through rustup, but maybe rustup could download it at the same time it downloads rust-docs?

[kinnison]

I think if we were to distribute the book we should simply bundle it into the release so that an update of rustup would get it. We could even compile it into rustup if we wanted to get particularly grungy.