If you think you have an useful/interesting scene you want to share with the rest of the world, just add it. On top of every Wikibooks page you should see a 'edit this page' link. Just click on it when you are in the 'YafRay: Scenes with Explanations' page and give your scene an appropriate place/name. After you save the page (always preview it first, of course), you can click on your scene name and put your scene on the empty page you are linked to.
If you are not sure about anything, just look at how other pages are made.