These are the sources for https://doc.rust-lang.org/nightly/unstable-book/.
To generate them, run ./x doc unstable-book, which will generate HTML files in build/host/doc/unstable-book using src/tools/rustbook.
If you need to change the overall structure, modify src/tools/unstable-book-gen/src/SUMMARY.md.
Note that most of this book is autogenerated by unstable-book-gen, with the exception of compiler-flags and compiler-environment-variables.
As a result, it does not integrate well with mdbook. Use ./x doc instead.