Update install and contributing guides to use the root justfile helpers
(`just fmt`, `just fix -p <crate>`, and targeted tests) instead of the
older cargo fmt/clippy/test instructions that have been in place since
459363e17b. This matches the justfile relocation to the repo root in
952d6c946 and the current lint/test workflow for CI (see
`.github/workflows/rust-ci.yml`).