When gitbook moved (rather poorly - there was a lot of confusion on what would happen to the old codebase) to a service, mdbook is what I switched over to, and I'm glad I had been forced to look for an alternative to gitbook.

Haven't really done anything with Rust, but having been bitten once, my research suggested they'd be unlikely to drop mdbook anytime soon.

Edit: example GitLab-hosted book: https://gitlab.com/strivinglife/book-git-commands