Make the examples/catalog instructions a bit less confusing:
* Make the README.md file provide explicit instructions about how to
run each example.
* Try to clarify that the lib/main.dart message is instructional and
not an error message.
Additionally, adjust examples/layers/lib/main.dart to be consistent
with examples/catalog/lib/main.dart.
Fixes#22640.
Testing Done:
* Ran `flutter run` from examples/catalog and from examples/layers,
saw the updated message.
