Merge pull request #251190 from alejandrosame/docs-mention-nrd

doc manuals: add reference to nixos-render-docs

doc/README.md

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 This directory houses the sources files for the Nixpkgs manual.
 
 You can find the [rendered documentation for Nixpkgs `unstable` on nixos.org](https://nixos.org/manual/nixpkgs/unstable/).
+The rendering tool is [nixos-render-docs](../pkgs/tools/nix/nixos-render-docs/src/nixos_render_docs), sometimes abbreviated `nrd`.
 
 [Docs for Nixpkgs stable](https://nixos.org/manual/nixpkgs/stable/) are also available.