feat: add allowed_hosts support in local_backend (#3805)

website/content/docs/beta-features.md

@@ -25,9 +25,11 @@ backend:
 # when using the default proxy server port
 local_backend: true
 
-# when using a custom proxy server port
 local_backend:
+  # when using a custom proxy server port
   url: http://localhost:8082/api/v1
+  # when accessing the local site from a host other than 'localhost' or '127.0.0.1'
+  allowed_hosts: ['192.168.0.1']
 ```
 
 4. Start your local development server (e.g. run `gatsby develop`).