[Bug] Markdown to Override API fails if it has multiple "---"

**Describe the bug**

I have a setup which uses the docfx.json "override" to augment API docs with additional markdown.
But I kept on getting errors like this:

```text
SomeFile.md: warning invalid-yaml-header: Exception during deserialization
```

After long research I found out that the problem is `---` (triple dashes) in the body of the file, used to create lines (`

---

`) tags. As soon as you have at least 2 in the body, docfx seems to get confused and thinks it's another **yaml header**.

This fails:

```markdown
---
uid: ToSic.Sys
summary: ToSic.Sys is for internal helpers and base classes which are just FYI.
---

Some content

---

## History

1. Introduced in 2sxc 15.0 as `ToSic.Lib` (previously was part of `ToSic.Eav`)
1. Changed to `ToSic.Sys` in 2sxc 19.0 to better reflect that it's the core system functionality.

---
```

While this works (note I removed one of the `---`).

```markdown
---
uid: ToSic.Sys
summary: ToSic.Sys is for internal helpers and base classes which are just FYI.
---

Some content

REMOVED THIS LINE

## History

1. Introduced in 2sxc 15.0 as `ToSic.Lib` (previously was part of `ToSic.Eav`)
1. Changed to `ToSic.Sys` in 2sxc 19.0 to better reflect that it's the core system functionality.

---
```

**To Reproduce**

1. Setup an override to get md files
2. Create the md-file with just the yml header, works
3. add content with tripple-dashes, once you add 2, it fails.

**Expected behavior**

Docfx should only treat the initial `---` block as yml header.

**Context (please complete the following information):**
- OS: Windows
- Docfx version: 2.78.4 and 2.78.5

**Workaround**

As of now, you can work around this by creating 6-dashes like `------` for separation lines.
But it took me hours to figure out what was going on.

Start with the docfx.json override path and the Markdown YAML-header handling that emits invalid-yaml-header; reproduce the issue using SomeFile.md and the two examples provided. Done means only the initial --- block is treated as the YAML header, while later --- separators remain body content without warnings.

Written by the indexing model from the issue text.