Improve shallow rendering example

It's still unclear for newbies what shallow rendering is. I think we could have a better example showing what it means, for instance, by testing a component that contains a child.

Happy to contribute to it later this week if that's ok 😄

Locate the current shallow rendering documentation and its existing example, then read how the example explains component boundaries. Add a clear child-component example that shows what shallow rendering includes and excludes, and confirm the documentation makes the behavior understandable to newcomers.

Written by the indexing model from the issue text.