I recently notices that Tex.SE has a Welcome to Tex.SX! meta post. In it, it describes what the site is and how to use it. It also includes information on the scope. I think this would be a good addition to PPCG. If a new user does not understand how to use the site properly, most users leave a comment saying "Welcome to PPCG.SE! Here are some ways to improve your post: ..." It would be a good idea to link a "Welcome to PPCG" meta post to those comments to help the new user understand how to post and some of our standard loopholes, for example.Edit:
As xnor suggested, this post would be the place where a simple guide to making questions and answers is visible, with suggestions on formatting, containing objective-winning critieria, etc.Edit 2:
I have drafted an example here based on the Tex.SE one. Ping me in chat with any comments/concerns.