The document hierarchy is now conveyed using the
`\startsectionlevel`/`\stopsectionlevel` by default. This makes it easy
to include pandoc-generated snippets in documents at arbitrary levels.
The more semantic environments "chapter", "section", "subsection", etc.
are used if the `--top-level-division` command line parameter is set to
a non-default value.
Closes: #5539