Versioning of the documentation

Hi!

Just noticed the 0.4 update, congrats on the milestone!

I had finished the book recently and was about to do a recap of what I had learned, when I noticed that my code no longer matched the code in the book. This led me to a moment of confusion and I had to dig a little here in the repo to fully understand what happened. In order to avoid this sort of confusion I think 2 things could be done.

1) Display the number of the version of bevvy (used in the book) next to the logo.
Why?
This would make it easier for people following the book to know if they should update their crate and expect changes. This is specially true if a new version introduces breaking changes.

2) Version the documentation
Why?
By having versioned documentation (with specific urls), you guarantee to your users that the doc will not change as they are going through it. This will become more and more relevant as this doc grows, and finding a solution for this problem is probably easier now than later.

Let me know what you think of this and if you' d like some help.

Cheers