Make addheader respect different YAML/Markdown sections

[mxmehl]

Follow-up from #230.

YAML or Markdown files may split their files in sections, separated by ---. We should investigate how addheader may mess with these files and whether adding comments before a first-line --- will break them.

We also have special Markdown files that support YAML (or also TOML) in their first section (Hugo refers to that as "front matter"), but pure Markdown (with HTML comments) in the following section:

---
title: My Title
author: Me

# This is a comment in yaml/toml/whatever inside of a Markdown file
---

# Chapter 1

Hello, world.

<!-- This is a comment inside of Markdown -->

Any ideas how we can cover the different cases without making everything too complex and overly specific?

[niccokunzmann]

I just had a look and reuse is happy with this:

---
# SPDX-FileCopyrightText: 2024 2024 Nicco Kunzmann and Open Web Calendar Contributors <https://open-web-calendar.quelltext.eu/>
#
# SPDX-License-Identifier: GPL-2.0-only

description: How to report security concerns about the Open Web Calendar project.
---

I would like it if reuse annotate is able to add the license header not as a comment but into the fontmatter automatically if this is given in a markdown file.

The comments before the first line definitely break the rendering.

Example with the comment:

<!--
SPDX-FileCopyrightText: 2024 2024 Nicco Kunzmann and Open Web Calendar Contributors <https://open-web-calendar.quelltext.eu/>

SPDX-License-Identifier: GPL-2.0-only
-->

---
description: How to report security concerns about the Open Web Calendar project.
---

# Security Policy

NOTE: This only seems to work if the frontmatter contains not only comments.

[mschoettle]

I am facing the same issue. reuse is also happy with the comments after the frontmatter. I.e., based on the above example:

---
description: How to report security concerns about the Open Web Calendar project.
---
<!--
SPDX-FileCopyrightText: ...

SPDX-License-Identifier: ...
-->

# Security Policy

I kinda like @niccokunzmann's version. We could try if the change in #378 could be updated to skip the first ---.

Update: I gave it a try in #1170 but it's only an ugly hack so far.