adding document mode

I've been thinking a lot about the treehouse and I feel like it's time to say goodbye to the tree format.

src/config.rs

@@ -152,6 +152,8 @@ impl Config {
     }
 
     pub fn page_url(&self, page: &str) -> String {
+        // We don't want .dj appearing in URLs, though it exists as a disambiguator in [page:] links.
+        let page = page.strip_suffix(".dj").unwrap_or(page);
         format!("{}/{}", self.site, page)
     }