Improve .cargo/config section in the Writing Shader Crates documentation

[newDINO]

1. Add git clone https://github.com/Rust-GPU/rust-gpu.git --depth 1 to the end of "Clone the rust-gpu repository"
2. Change "cargo build --release in rust-gpu" into "cargo build -r -p rustc_codegen_spirv in rust-gpu"

These small changes may reduce the downloading and compiling time as well as space occupation for new users.

[Firestar99]

Most of the docs are sadly very outdated right now, in part cause we're starting to replace most ways of running the compiler, with cargo-gpu slowly turning into the new de-facto way. While it progressed far already, it is not yet ready for a full release.

I'm not even sure the Using `.cargo/config.toml` way even works anymore.