docs: Pin mdbook to v0.4.40 (#35380)

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

crates/docs_preprocessor/Cargo.toml

@@ -9,7 +9,9 @@ license = "GPL-3.0-or-later"
 anyhow.workspace = true
 command_palette.workspace = true
 gpui.workspace = true
-mdbook = "0.4.40"
+# We are specifically pinning this version of mdbook, as later versions introduce issues with double-nested subdirectories.
+# Ask @maxdeviant about this before bumping.
+mdbook = "= 0.4.40"
 regex.workspace = true
 serde.workspace = true
 serde_json.workspace = true