Document the Gradle plugin's filterClasspath property/closure

[oliveryasuna]

I originally opened vaadin/flow#19528, before realizing there is an undocumented feature to filter the classpath scanning. This feature should probably be documented at https://github.com/vaadin/docs/blob/latest/articles/guide/start/gradle.adoc.

Vaadin team, is there any reason why it is not? If there is not, I am happy to contribute a PR.

[oliveryasuna]

#3479

[jouni]

Thanks for taking the initiative and sending the pull request 👍