Move the Rust tutorial into a sub-directory

To make room for the C++ version next to it :-)

docs/tutorial/rust/src/main_memory_tile.rs

@@ -0,0 +1,32 @@
+/* LICENSE BEGIN
+    This file is part of the SixtyFPS Project -- https://sixtyfps.io
+    Copyright (c) 2020 Olivier Goffart <olivier.goffart@sixtyfps.io>
+    Copyright (c) 2020 Simon Hausmann <simon.hausmann@sixtyfps.io>
+
+    SPDX-License-Identifier: GPL-3.0-only
+    This file is also available under commercial licensing terms.
+    Please contact info@sixtyfps.io for more information.
+LICENSE END */
+#[allow(dead_code)]
+fn main() {
+    MainWindow::new().run();
+}
+sixtyfps::sixtyfps! {
+// ANCHOR: tile
+MemoryTile := Rectangle {
+    width: 64px;
+    height: 64px;
+    background: #3960D5;
+
+    Image {
+        source: @image-url("icons/bus.png");
+        width: parent.width;
+        height: parent.height;
+    }
+}
+
+MainWindow := Window {
+    MemoryTile {}
+}
+// ANCHOR_END: tile
+}