docs: add examples folder

examples/shell-environments/README.md

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+# shell-environment
+
+This example shows how to create a shell environment which
+includes a diverse set of tools:
+
+```
+terraform
+wget
+bat
+git
+```
+
+You can search for more package in [nix packages](https://search.nixos.org/packages)
+
+## Usage
+
+The [`devShells` option](https://flake.parts/options/flake-parts.html#opt-perSystem.devShells) is used by the following command:
+
+```sh
+nix develop
+```
+
+You can have as many shells as you want, in this [flake.nix](./flake.nix), you also have
+`another_env` which includes `curl`. To open it:
+
+```sh
+nix develop .#another_env
+```

examples/shell-environments/flake.nix

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+{
+  description = "Description for the project";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs = inputs@{ flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
+      systems = [ "x86_64-linux" "aarch64-darwin" "x86_64-darwin" ];
+      perSystem = { config, self', inputs', pkgs, system, ... }: {
+        devShells.default = pkgs.mkShell {
+          nativeBuildInputs = with pkgs; [ terraform wget bat git ];
+        };
+
+        devShells.another_env = pkgs.mkShell {
+          nativeBuildInputs = with pkgs; [ curl ];
+        };
+      };
+
+    };
+}