Migrate "Extension Reference" wiki page

Created by: seancolsen

Reviews welcome. Will merge in 5 days if no comments.

This page migration really ballooned into way bigger of a project than I wanted. I ended up researching quite a bit about XSD and RELAX NG in hopes that I could generate some good-looking docs from an .xsd or .rng file. But unfortunately the available tools for turning schema definition into doc are not good. In the process of this research, however, I did learn a bit more about how people typically describe XML schemata. Even though this page was a lot of work to put together, I think this format is ultimately a bit superior to the database schema definition page. It is a bummer that the pages are in different formats. I wrestled with this a bit. Anyway, there's lots more I could say about all this, but like I said, I've already spent too much time on this one. Phew!