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refactor(root): relocate the contributing guide into .github/ #382

Job	Run time
rust-ci / Detect Cargo.toml	37s
rust-ci / Cargo check + clippy + fmt	0s
rust-ci / Cargo audit (security)	0s
rust-ci / Cargo test	-1s
rust-ci / llvm-cov line coverage	-1s
	35s