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
d832bd0cc2. This matches the justfile relocation to the repo root in
a3bb06866 and the current lint/test workflow for CI (see
`.github/workflows/rust-ci.yml`).