Add Element::into_document

It's more convenient in some cases and also makes the example code less
"misleading": Now the string represents a full, correct HTML document.

README.md

@@ -25,6 +25,7 @@ let page: String = html((
         p(("This is an example for a ", em("simple"), " web page.")),
     )),
 ))
+.into_document()
 .render_to_string()
 .unwrap();
 ```