Document geometry module and replace zero method with default (#3515)

Nothing earth-shattering here, but all our geometry types are now fully
documented.

Release Notes:

- N/A

crates/gpui2/src/window.rs

@@ -1183,7 +1183,7 @@ impl<'a> WindowContext<'a> {
                 }
 
                 let available_space = cx.window.viewport_size.map(Into::into);
-                root_view.draw(Point::zero(), available_space, cx);
+                root_view.draw(Point::default(), available_space, cx);
             })
         });