Add examples of child collections

[bjarnef]

The docs mention child collections using AddChildCollectionGroup() and AddChildCollection() but not really any examples of how these are used:
https://docs.umbraco.com/umbraco-ui-builder/v/13.ui-builder.latest-lts/collections/child-collections

It also mention a StudentProjectController to retrieve child collections, but I don't quite see how this is used and where does it get projectId parameter from?
https://docs.umbraco.com/umbraco-ui-builder/v/13.ui-builder.latest-lts/collections/child-collections/retrieve-child-collections