Add a nix flake check job. Add comments on CI jobs. #212

Job	Run time
cargo (cargo fmt --verbose --all -- --check)	1m 5s
nix-flake-check (ubuntu-latest)	22s
nix-fmt-check	19s
cargo (cargo check --locked --workspace)	3m 23s
nix-flake-check (macos-latest)	1m 34s
cargo (cargo check --locked --examples --workspace)	3m 17s
cargo (cargo test --locked --lib --bins --all-features --workspace)	6m 28s
cargo (cargo doc --locked --all-features --workspace)	6m 23s
cargo (cargo test --locked -p nannou_core --no-default-features --features "libm serde")	1m 18s
cargo (cargo build --locked -p nannou --features "wasm-experimental" --target wasm32-unknown-unkn...	2m 10s
cargo (mdbook build guide/)	1m 6s
cargo (cargo test --locked -p book_tests)	3m 30s
verifications	1s
cargo-publish	0s
guide-push-to-deploy	0s
	30m 56s

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