What should go in world.launch file?
The text is the same as the title.
I don't know what world.launch in number 9 of the homepage above means and what it contains. If anyone knows about this, please reply.
Your question is not clear. Please add more details about the question such as what are you trying to do, what is the problem you are facing etc from next time. So everyone don't have to read through the whole article that you have provided. You can edit the question using the pen like button in the right bottom corner of the question.
If you are looking for the simple world.launch file, here is the tutorial