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?