My org has used mdBook: https://rust-lang.github.io/mdBook/ (That link is itself a rendered mdBook, so that'll give you an idea of the feature set.)
(While it's definitely a Rust "thing", if you just have a set of .md files, all you need is a "SUMMARY.md" (which contains the ToC) and a small config file; i.e., you don't have to have any Rust code to use it, and it works fine without. We document a large, mostly non-Rust codebase with it.)
(While it's definitely a Rust "thing", if you just have a set of .md files, all you need is a "SUMMARY.md" (which contains the ToC) and a small config file; i.e., you don't have to have any Rust code to use it, and it works fine without. We document a large, mostly non-Rust codebase with it.)