docs: add codegen examples

[skyzh]

• close VSCode or other tools that might run rust-analyzer before you start.
• move original source code into raytracer folder
• create raytracer_codegen crate
• add raytracer_codegen to dependencies of raytracer, copy all codes from tutorial
• use procedural macro in raytracer
• create a root Cargo.toml with workspace definition
• before commit, MAKE SURE you didn't accidentally add target/ folder or other binaries into git (you may add them into .gitignore)