Add /to short links for et and engine-tool. (#11344)

I.e. to make
https://github.com/flutter/engine/blob/main/tools/engine_tool/README.md
more discoverable/linkable.

firebase.json

@@ -677,8 +677,10 @@
       { "source": "/to/edit-plugins", "destination": "/packages-and-plugins/developing-packages#edit-plugin-package", "type": 301 },
       { "source": "/to/editor-setup", "destination": "/get-started/editor", "type": 301 },
       { "source": "/to/engine-rbe", "destination": "https://github.com/flutter/engine/blob/main/docs/rbe/rbe.md", "type": 301 },
+      { "source": "/to/engine-tool", "destination": "https://github.com/flutter/engine/blob/main/tools/engine_tool/README.md", "type": 301 },
       { "source": "/to/engine-tool-bug", "destination": "https://github.com/flutter/flutter/issues/new?labels=e:%20engine-tool", "type": 301 },
       { "source": "/to/endorsed-federated-plugin", "destination": "/packages-and-plugins/developing-packages#endorsed-federated-plugin", "type": 301 },
+      { "source": "/to/et", "destination": "https://flutter.dev/to/engine-tool", "type": 301 },
       { "source": "/to/federated-plugins", "destination": "/packages-and-plugins/developing-packages#federated-plugins", "type": 301 },
       { "source": "/to/ffi-package", "destination": "/packages-and-plugins/developing-packages#plugin-ffi", "type": 301 },
       { "source": "/to/flutter-plugins-configuration", "destination": "/release/breaking-changes/flutter-plugins-configuration", "type": 301 },