Make examples/catalog instructions a bit less confusing (#24252)

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.