Add a “you are viewing an older version of the book” banner

[xrisk]

With reference to the on-going efforts in rust-lang/rust#18558, rust-lang/rust#9461; as well as a bunch of other issues complaining about out-dated results in Google Search, would it make sense to add a warning banner for older versions of the book? Like Django does for example?

Apologies if creating a new issue wasn’t an appropriate thing to do.

[steveklabnik]

Apologies if creating a new issue wasn’t an appropriate thing to do.

It's all good.

Anything we do for those issues is also going to take care of the book, that is, it has to address doc.rust-lang.org as a whole. So I'm gonna give this a close, not because this isn't a good idea, but because we don't really need yet another issue tracking this :) Thanks!