Grouping items under headers in the AST

Howdy!

For hierarchical document models it can be very useful to be able to access an AST where the items are nested under their corresponding sections / headers as this eliminates a fairly painful windowing step during AST decomposition in other frameworks like Docling

I've offered a subclasses parser to provide this here docling-project/docling#1893 (comment) that I'm still playing with but would love if it was an option out of the box