docs: document how to override mkShell/stdenv for devShell (#709) #1229

Job	Run time
examples-darwin	2m 12s
tests (https://releases.nixos.org/nix/nix-2.18.2/install)	2m 33s
check-fmt	37s
tests (https://releases.nixos.org/nix/nix-2.18.2/install, --override-input nixpkgs $(./ci/ref-fro...	2m 32s
check-dead-code	21s
examples-linux (alt-registry)	1m 28s
examples-linux (build-std)	1m 29s
examples-linux (cross-musl)	1m 25s
examples-linux (cross-rust-overlay)	1m 25s
examples-linux (cross-windows)	1m 28s
examples-linux (custom-toolchain)	1m 24s
examples-linux (end-to-end-testing)	1m 31s
examples-linux (quick-start)	1m 23s
examples-linux (quick-start-simple)	1m 26s
examples-linux (quick-start-workspace)	1m 25s
examples-linux (sqlx)	1m 21s
examples-linux (trunk-workspace)	1m 35s
examples-linux (trunk)	1m 23s
all systems go	0s
	26m 58s

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