Increased compatability when front matter is used

[thompson-tomo]

I want the processor to automatically exclude front matter content and instead only process the actual content.

What this would look like is if the first line is --- it should locate the next --- and start from there.

An example of the expected behaviour would be

---
# comment
---

# header

## sub-header

Should result in a output with 2 lines in the toc aka

---
# comment
---

**Table of contents**
- header
  - sub-header

# header

## sub-header

See markdown spec. https://www.markdownlang.com/advanced/frontmatter.html

[thompson-tomo]

With latest version this is not an issue in update-only mode but only when adding it to all docs. As such should be addressed along with #153