Improve REST api getting started guide

support/doc/api/quickstart.md

@@ -14,10 +14,20 @@ There are several ways to know if a website uses the PeerTube software:
 
 Some endpoints need authentication. We use OAuth 2.0 so first fetch the client tokens:
 
-```bash
+::: code-group
+
+```bash [Curl]
 curl https://peertube.example.com/api/v1/oauth-clients/local
 ```
 
+```js [JS]
+const { client_id, client_secret } = await fetch('https://peertube.example.com/api/v1/oauth-clients/local', {
+  headers: { 'Content-Type': 'application/json' }
+}).then(res => res.json())
+```
+
+:::
+
 Response example:
 
 ```json
@@ -31,12 +41,29 @@ Response example:
 
 Now you can fetch the user token:
 
-```bash
+::: code-group
+
+```bash [Curl]
 curl -X POST \
  -d "client_id=v1ikx5hnfop4mdpnci8nsqh93c45rldf&client_secret=AjWiOapPltI6EnsWQwlFarRtLh4u8tDt&grant_type=password&response_type=code&username=your_user&password=your_password" \
   https://peertube.example.com/api/v1/users/token
 ```
 
+```js [JS]
+const { access_token } = await fetch('https://peertube.example.com/api/v1/users/token', {
+  contentType: 'application/x-www-form-urlencoded',
+  method: 'POST',
+  body: new URLSearchParams({
+    client_id: '...',
+    client_secret: '...',
+    username: '...',
+    password: '...',
+    grant_type: 'password'
+  })
+}).then(res => res.json())
+```
+:::
+
 Response example:
 
 ```json