Add platform independent sample script for headless init

src/pages/cli/usage/headless.mdx

@@ -177,6 +177,54 @@ Installs, initializes, and provisions resources for a sample amplify application
 If the repository contains a `yarn.lock` and/or `package.json` file, the sample will be installed with the corresponding package manager and started after resources have been provisioned.
 
 ### Sample script
+```javascript
+const shell = require('shelljs');
+const amplify = require('./amplify-headless-init-payload.json');
+const cmd =
+  'amplify init --amplify "' +
+  JSON.stringify(amplify.amplify).replaceAll('"', '\\"') +
+  '" --frontend "' +
+  JSON.stringify(amplify.frontend).replaceAll('"', '\\"') +
+  '" --providers "' +
+  JSON.stringify(amplify.providers).replaceAll('"', '\\"') +
+  '" --yes';
+
+if (shell.exec(cmd).code !== 0) {
+  shell.echo('Error amplify init');
+  shell.exit(1);
+}
+```
+
+amplify-headless-init-payload.json content
+```json
+{
+  "amplify": {
+    "projectName": "headlessProjectName",
+    "envName": "myenvname",
+    "defaultEditor": "code"
+  },
+  "frontend": {
+    "frontend": "javascript",
+    "framework": "react",
+    "config": {
+      "SourceDir": "src",
+      "DistributionDir": "build",
+      "BuildCommand": "npm run-script build",
+      "StartCommand": "npm run-script start"
+    }
+  },
+  "providers": {
+    "awscloudformation": {
+      "configLevel": "project",
+      "useProfile": false,
+      "profileName": "default"
+    }
+  }
+}
+
+```
+
+#### Sample bash script for Unix platform
 
 ```bash
 #!/bin/bash