8f14bf4712
the Netlify `_redirects` file must be in the root directory [0] of the
files to serve, and mdBook copies all the files in `src` that aren't
`.md` to the output directory [1].
[0]: https://docs.netlify.com/routing/redirects/
[1]: https://rust-lang.github.io/mdBook/guide/creating.html#source-files
(cherry picked from commit 2d74b56aee)