Docs: explain breaking long lines in Groovy

[Gitkbc]

Fixes #64

This PR enhances the Groovy documentation by adding guidance on how to
break long lines into multiple lines, which is helpful when addressing
linter warnings such as LineLength.

The documentation includes a clear example showing how collections can
span multiple lines, improving readability and maintainability.

If this content would be better placed in an existing documentation
section or requires adjustment to match project conventions, I’m happy
to update it based on maintainer feedback.

[paulk-asert]

Thanks for looking into this. There are no doubt many places where the doco can be improved. Most of the documentation that might change between Groovy versions is kept in the groovy repo itself not this one. Perhaps style-guide.adoc and/or core-syntax.adoc and/or core-differences-java.adoc in that repo might be better places to consider changes. For long strings, Groovy's various multi-line strings and related method calls are generally preferred to concatenated strings. I would also expect to see some discussion about repeated method calls and expressions with operators.