At the bottom of the CD article when you're viewing it, click "options", and then click "parent", and then enter the page you want the article to go under. So in this case it'd be "series", but the parent article has to be created first, so if there's no "series" article yet, you create that, set the parent to "spin-offs", and then go to the CD page and set the parent to "series."
Also, to get the pagetree to display on the "series" page, you just need to put in this line of code on the page:
[[module PageTree showRoot="true"]]