Add NixOS installation instructions to docs

[lunaneff]

On NixOS, there's an easy method of modifying packages to for example add extra patches. I've used this to add the intro skip button to jellyfin-web, instead of downloading your prebuilt version:

{ pkgs, ... }:

{
  services.jellyfin = {
    enable = true;
    package = pkgs.jellyfin.override {
      jellyfin-web = pkgs.jellyfin-web.overrideAttrs (oldAttrs: {
        patches = [
          (pkgs.fetchpatch {
            url = "https://github.com/jellyfin/jellyfin-web/compare/v${oldAttrs.version}...ConfusedPolarBear:jellyfin-web:intros.diff";
            hash = "sha256-qm4N4wMUFc4I53oQJUK1Six0cahVYz3J+FgO2vvSvXM=";
          })
        ];
      });
    };
  };
}

It'd probably be useful to others as well if this was included in the docs, as the way NixOS is designed makes the listed method of manually overwriting the webui files impossible. Or if you'd prefer to avoid instructions for specific distros, it'd at least be helpful if you could include the URL to GitHub's auto-generated patch file.