url_info is a method used with the `info` command. Page assets have configurable urls. For example, you might have an rss feed at `assets/pages/feed.xml` but you could set the url configuration for that page so that it would have a destination of `_site/feed/index.xml`. The `url_info` method shows what the destination url will be. Doc pages don't have any such configuration but they extend the page asset, so I set the `url_info` to an empty string so it won't show up in the output of info.

There may be a better way to do this, or maybe a better pattern of extending assets so I don't have to override unused methods.