Add a node group example in example.dae

[akiooo45]

Background

Checklist

• [ ] The Pull Request has been fully tested
• [ ] There's an entry in the CHANGELOGS
• [ ] There is a user-facing docs PR against https://github.com/daeuniverse/dae

Full Changelogs

• [Implement ...]

Issue Reference

Closes #[issue number]

Test Result

[sumire88]

Looks like this one is duplicated with #488