Add installation instructions for Nix

[orhun]

Is there an existing issue or pull request for this?

• I have searched the existing issues and pull requests

Feature description

It would be nice to add instructions for Nix, similar to other installation docs, e.g. https://git-cliff.org/docs/installation/arch-linux

Desired solution

Add Nix.md under website/docs/installation

Alternatives considered

None.

Additional context

See https://search.nixos.org/packages?channel=24.05&show=git-cliff&from=0&size=50&sort=relevance&type=packages&query=git-cliff

[0x61nas]

Hi, I would love to work on this, please assign this issue to me if there's no one are already working on it

[orhun]

sir, you got it