Book: missing "next chapter" button on the main page

If you open [book.async.rs] without the [`/introduction.html`][2] part, the "Introduction" chapter will not be highlighted and the "next page" button on the right side will not be available.

Most links go to the main page (with no chapter selected), so this results in an extra click every time the book is opened. And it's arguably even more confusing to beginners (like me 🙂) who are not yet used to this quirk.

This seems to be a problem with mdBook, but their [main page][3] is fine. Perhaps the bug has already been fixed in a newer version?

[book.async.rs]: https://book.async.rs/
[2]: https://book.async.rs/introduction.html
[3]: https://rust-lang.github.io/mdBook/

Start by opening book.async.rs and comparing its main page with the linked mdBook main page, then inspect the repository's mdBook configuration and version. Confirm whether the missing Introduction highlight and next-page button are caused by an outdated mdBook release or configuration; done means the main page opens with the Introduction selected and the next-page button available.

Written by the indexing model from the issue text.