tutorial: separate out the rust code into .rs files

That way even the "incomplete" snippets can be compile tested as only
the anchors will be pulled into the docs but the complete file can be
compile-tested in the future.

docs/tutorial/src/main_tiles_from_rust.rs

@@ -0,0 +1,24 @@
+fn main() {
+    use sixtyfps::Model;
+
+    let main_window = MainWindow::new();
+
+    // Fetch the tiles from the model
+    let mut tiles: Vec<TileData> =
+        main_window.get_memory_tiles().iter().collect();
+    // Duplicate them to ensure that we have pairs
+    tiles.extend(tiles.clone());
+
+    // Randomly mix the tiles
+    use rand::seq::SliceRandom;
+    let mut rng = rand::thread_rng();
+    tiles.shuffle(&amp;mut rng);
+
+    // Assign the shuffled Vec to the model property
+    let tiles_model =
+        std::rc::Rc::new(sixtyfps::VecModel::from(tiles));
+    main_window.set_memory_tiles(
+        sixtyfps::ModelHandle::new(tiles_model.clone()));
+
+    main_window.run();
+}