The cxx.rs docs site TOC is broken

[nyurik]

To reproduce:

• navigate to https://cxx.rs/
• click the hamburger menu (upper left corner), and select 5.1 Cargo
• again, click on that menu and select another build method like 5.2 Bazel

Result - it tries to open https://cxx.rs/build/build/bazel.html (duplicate prefix)

[dtolnay]

This bisected to rust-lang/mdBook#2473. I have fixed it by pinning mdbook to v0.4.41 but still need to report a bug upstream.