docs: move the Nix install block from README into nix.md (#76)
Nathan wants Nix docs out of the READMEs into a dedicated nix.md across the
convoy/pty ecosystem; pty is the only core repo with Nix doc content, so this
is a pty-only change.
Move the "Or with Nix:" install block (nix profile install + nix develop) out
of README.md into a new nix.md at the repo root, and leave a one-line pointer
("Or install with Nix — see nix.md.") where the block was. nix.md structures
the Nix install (profile install / nix run) and the dev shell (nix develop +
the clone workflow), and notes flake.nix is the source of truth. Every command
is accurate to the flake — packages.default has meta.mainProgram = "pty" so
`nix run` works, and the dev shell provides nodejs_22/python3/pkg-config.
Doc-only and reversible: flake.nix is untouched.