Some books on the Rust bookshelf show the rust version and exact commit from which the documents were built.

For example, see the bottom-right corner of these pages:

• https://doc.rust-lang.org/stable/book/
• https://doc.rust-lang.org/beta/book/
• https://doc.rust-lang.org/nightly/book/

What's the best way to do that in mdbook land? Is it possible at all to do so automatically?

If we need to build something for it, how about adding a version/commit signature to the bottom of the left-hand sidebar?