Update Kani Book

[carolynzech]

Add the following to the book:

• Reference section on contracts. The point of this section is not to be an exhaustive explanation--we have good documentation on contracts already, and we don't want to maintain it in two places. The goal of the section is to give readers an intuition for the main idea and then direct them to more detailed resources.
• Add a reference to the blog--it's a good resource to learn about Kani and it would be nice to have an easier way to find it. Inspired by this post.
• Move the crates documentation earlier. This partly addresses #3029 by at least making our crates documentation more visible in the book. (I don't have this PR as resolving that issue, since we would need to actually publish the docs on crates.io to do that.)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[zhassan-aws]

Thank you!