Render RBE to PDF with `mdbook-pandoc`

Hey there,
@max-heller was a great help to comprehensive-rust and rust-patterns as they helped us set up rendering to PDF. I found these issues: https://github.com/rust-lang/rust-by-example/issues?q=is%3Aissue+pdf+is%3Aclosed stating that we should open another one with new information, when they come available. So here we are.

mdbook-pandoc has some pre-rendered examples in also of Rust-by-example and the book: https://github.com/max-heller/mdbook-pandoc?tab=readme-ov-file#rendered-books

• Comprehensive Rust: https://google.github.io/comprehensive-rust/comprehensive-rust.pdf

• Rust Design Patterns: https://rust-unofficial.github.io/patterns/rust-design-patterns.pdf

(At this point, I think it makes sense to write up a condensed tutorial how to add PDF support to an existing mdbook...)

Some Resources

• Initial support PR: https://github.com/rust-unofficial/patterns/pull/394

• Adding download icon to menu bar: https://github.com/rust-unofficial/patterns/commit/d96de404c45f17310c3eb77b4669d715ce923d33

• Line wrapping in code blocks: https://github.com/rust-unofficial/patterns/commit/3dd157b9d751e2a5a391b2933ae6eb8467ea1eaf

• Readme of mdbook-pandoc: https://github.com/max-heller/mdbook-pandoc?tab=readme-ov-file#getting-started

Start with the mdbook-pandoc README's getting-started section and compare the referenced Rust Design Patterns support PR and commits. Determine how Rust by Example should be rendered and how the PDF should be exposed, then verify that the generated book and download entry work as intended.