Generate TOC improvements

[keynmol]

Now that #17 is merged, here's how it can be improved:

1. If the document has a *single level-1 heading (=title), the TOC should be inserted right under it

   This means that the entire TOC will have to be shifted one level down (heading 2 becomes heading 3, etc.) and the original heading-1 is not rendered at all

2. If the document has a YAML frontmatter, the TOC should be inserted under it (unless rule 1 applies)

   This would require to

   • Enable YAML parsing on Markdig
   • Propagate the location of yaml block all the way to Document Index

[artempyanykh]

if the document has a *single level-1 heading (=title), the TOC should be inserted right under it

Yes! 🙌 Also, would be great to insert an extra newline between the title and the toc.