docs(nix): add installation instructions for Nix (#669)

website/docs/installation/nix.md

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+---
+sidebar_position: 4
+---
+# Nix
+
+If you are using Nix, **git-cliff** can be installed from the [official latest staple channel][channel-link].
+
+## Using nix-shell
+
+To temporarily install git-cliff in a shell environment, run:
+
+```sh
+nix-shell -p git-cliff
+```
+
+## Using nix-env
+
+To install git-cliff permanently, use:
+
+```sh
+nix-env -iA nixpkgs.git-cliff
+```
+
+> [!WARNING]
+> Using nix-env permanently modifies a local profile of installed packages.
+> This must be updated and maintained by the user in the same way as with a traditional package manager, 
+> foregoing many of the benefits that make Nix uniquely powerful. 
+> Using nix-shell or a NixOS configuration is recommended instead.
+
+
+## The new CLI
+
+If you're using the new experimental CLI, you can use any of the following:
+
+### `nix run
+```sh
+nix run nixpkgs#git-cliff
+```
+
+### `nix shell`
+
+To open a new shell with git-cliff available, use:
+
+```sh
+nix shell nixpkgs#git-cliff
+```
+
+## Using the Unstable Channel
+
+While new releases of **git-cliff** typically become available in the stable channel relatively quickly, you can use the unstable channel if you want to access the latest features and updates sooner.
+
+To add and update the unstable channel, run:
+
+```sh
+nix-channel --add https://nixos.org/channels/nixpkgs-unstable
+nix-channel --update nixpkgs
+```
+
+After updating to the unstable channel, you can use any of the previous installation commands to install **git-cliff**.
+
+
+[channel-link]: https://search.nixos.org/packages?channel=24.05&show=git-cliff&from=0&size=50&sort=relevance&type=packages&query=git-cliff