Improve Groovy instance building example for geometry properties

[florianesser]

For geometry properties the instance building example does not show a structure or a proper example how to assign a geometry.

[stempler]

It's a general problem that groups are not supported. I think for the source structure there could be the same issue

As the example is generated from the selection that's IMHO a programming and not a documentation task.

[github-actions[bot]]

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