Backfill missing details (restart/restartSec in full example,
gateway.pid, 0750 permissions, docker inspect commands) into
the canonical website/docs/getting-started/nix-setup.md and
delete the old standalone file.
Add comprehensive Nix documentation to the Docusaurus site at
website/docs/getting-started/nix-setup.md, covering nix run/profile
install, NixOS module (native + container modes), declarative settings,
secrets management, MCP servers, managed mode, container architecture,
dev shell, flake checks, and full options reference.
- Register nix-setup in sidebar after installation page
- Add Nix callout tip to installation.md linking to new guide
- Add canonical version pointer in docs/nixos-setup.md
