This PR pins `mdbook` to v0.4.40 to fix an issue with sidebar links
having some of their path segments duplicated (e.g.,
`http://localhost:3000/extensions/extensions/developing-extensions.html`.

For reference:

-
https://zed-industries.slack.com/archives/C04S5TU0RSN/p1745439470378339?thread_ts=1745428671.190059&cid=C04S5TU0RSN
-
https://zed-industries.slack.com/archives/C04S5TU0RSN/p1753922478290399

Release Notes:

- N/A