docs: Adding docs about CORS (#867)

Co-authored-by: Scarlett Eliza <[email protected]>

Author: erickzanardo

docs/docs/advanced/cors.md

@@ -0,0 +1,43 @@
+---
+sidebar_position: 8
+title: ⚔️  Handling Cross-Origin Resource Sharing (CORS)
+---
+
+Cross-Origin Resource Sharing or CORS is a common thing that needs to be handled by backend servers.
+This guide shows how the handling of CORS can be done in a Dart Frog project.
+
+:::warning
+To learn more about what CORS is, check out this [helpful documentation from MDN](https://developer.mozilla.org/docs/Web/HTTP/CORS).
+:::
+
+[Shelf](https://pub.dev/packages/shelf) already has a package in its ecosystem that handles CORS.
+Since Dart Frog is built on top of shelf, we can use it in Dart Frog as well. First add `shelf_cors_headers` to your project:
+
+```bash
+dart pub add shelf_cors_headers
+```
+
+Then, to allow CORS in routes, the following middleware can be created:
+
+```dart
+import 'dart:io';
+
+import 'package:dart_frog/dart_frog.dart';
+import 'package:shelf_cors_headers/shelf_cors_headers.dart' as shelf;
+
+Handler middleware(Handler handler) {
+  return handler
+      .use(requestLogger())
+      .use(
+        fromShelfMiddleware(
+          shelf.corsHeaders(
+            headers: {
+              shelf.ACCESS_CONTROL_ALLOW_ORIGIN: 'https://myfrontendurl.com',
+            },
+          ),
+        ),
+      );
+}
+```
+
+For a full example of a Dart Frog server that handles CORS, check [Very Good Hub](https://github.com/VeryGoodOpenSource/very_good_hub/tree/main/api).