nixos/nextcloud: update docs

- `binwalk` was updated to 3.1.0, which has been rewritten in rust. The python module is no longer available.

  See the release notes of [3.1.0](https://github.com/ReFirmLabs/binwalk/releases/tag/v3.1.0) for more information.

- The latest available version of Nextcloud is v31 (available as `pkgs.nextcloud31`). The installation logic is as follows:

  - If [`services.nextcloud.package`](#opt-services.nextcloud.package) is specified explicitly, this package will be installed (**recommended**)

  - If [`system.stateVersion`](#opt-system.stateVersion) is >=24.11, `pkgs.nextcloud30` will be installed by default.

  - If [`system.stateVersion`](#opt-system.stateVersion) is >=24.05, `pkgs.nextcloud31` will be installed by default.

  - Please note that an upgrade from v29 (or older) to v31 directly is not possible. Please upgrade to `nextcloud30` (or earlier) first. Nextcloud prohibits skipping major versions while upgrading. You can upgrade by declaring [`services.nextcloud.package = pkgs.nextcloud30;`](options.html#opt-services.nextcloud.package).

- `pkgs.nextcloud28` has been removed since it's out of support upstream.

- `buildGoModule` now passes environment variables via the `env` attribute. `CGO_ENABLED` should now be specified with `env.CGO_ENABLED` when passing to buildGoModule. Direct specification of `CGO_ENABLED` is now redirected by a compatibility layer with a warning, but will become an error in future releases.

  Go-related environment variables previously shadowed by `buildGoModule` now results in errors when specified directly. Such variables include `GOOS` and `GOARCH`.

    package = mkOption {

      type = types.package;

      description = "Which package to use for the Nextcloud instance.";

      relatedPackages = [ "nextcloud29" "nextcloud30" ];

      relatedPackages = [ "nextcloud29" "nextcloud30" "nextcloud31" ];

    };

    phpPackage = mkPackageOption pkgs "php" {

      example = "php82";