POST method returns 415
Here is some example raw data you should be sending (don't send it as form data):

{
  "type":[{"target_id":"article"}],
  "title":[{"value":"Hello World"}]
}


You do not need to send the Accept header, because that no longer matters because of the _format=json query string. And in this case, when successful, it will return a 201 so you can disregard the _format query string.

The POST to ?q=entity/node is correct, but don't forget to send an X-CSRF-Token header with a value that can be retrieved here: ?q=rest/session/token