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
Fixes#2609.
This PR introduces the new-style section headings: `\section[my-header]{My Header}` -> `\section[title={My Header},reference={my-header}]`.
On top of this, the ConTeXt writer now supports the `--section-divs` option to write sections in the fenced style, with `\startsection` and `\stopsection`.
