If there's no header defined, no sub headers are added as links to the TOC

[FransBouma]

E.g. if you have

(some image link)

## topic 1

## topic 2

these won't show in the ToC. If a header is added with ======, they are.

[FransBouma]

Likely caused by: https://github.com/FransBouma/DocNet/blob/master/src/DocNet/NavigatedPath.cs#L85

As when there's no header there's likely no root container added.
Or: https://github.com/FransBouma/DocNet/blob/master/src/MarkdownDeep/Extensions.cs#L9
where the hierarchy is created and thus whether there's a rootnavigator container.