Building - Practice Exercise Docs: Clarify how append file is rendered

[junedev]

This is regarding this section: https://exercism.org/docs/building/tracks/practice-exercises#h-file-docs-instructions-append-md

We had a lot of issues because people were not aware that the h1 header is removed and that the append content is rendered directly below the original instructions without any kind of separation. Usually you want to use an h2 after the h1 because of that so the content structure makes sense in end. Would be good to clarify how it works. Ideally with screenshots that illustrates it.