Consider including parts of the solution instead of copy pasting to the markdown file

[bjoernQ]

It might be better to use https://rust-lang.github.io/mdBook/format/mdbook.html#including-portions-of-a-file (especially with anchors) instead of copy-pasting code.

That way we could make sure that what is written in the book is always in sync with the actual code and we can make sure the code snippets are correct (since we check them in CI).

Only drawback I see when doing this: the ANCHOR comments in Rust source files might look a bit confusing - but we could explain that in the introduction