have custom appendix sections work in formats besides html

Feature requested

Currently, the the custom appendix mechanism only works for HTML. It would be nice if it also worked for, say, pdf and docx. At the moment {.appendix} has no effect whatsoever in these formats.

Current behaviour

---
title: "Document en français avec annexes"
format: pdf
lang: fr
toc: true
number-sections: true
---

## Un problème intéressant

Une description

## Plus de détail {.appendix}

Des détails

## Un problème compliqué

Des compléments

## Encore plus de détail {.appendix}

Plein de détails

-> No section is moved nor considered part of the appendix.

Whised-for behavior

I would expect uniform behaviour across html, pdf and docx formats, that is all the appendices to be sent at the end of the document and be numbered together.

Start by reproducing the embedded .qmd example in HTML, PDF, and DOCX output, focusing on how sections marked {.appendix} are handled. Compare the generated section placement and numbering across formats; done means appendix sections move to the end and are numbered together consistently.