Evaluate if we actually want to have headings within other elements

## Context

As of now we need to support headings within other element e.g. a dropdown because there are asciidoc pages with the this structure

See https://github.com/elastic/docs-builder/issues/285#issuecomment-2615798422 for additional context

## Goal

Understand use-cases for such structures and try to find out if we can or if we should only support top-level headings.