Improve chunking to always include parent header

[bsquizz]

When splitting chunks, include the header information in each chunk -- even if the content beneath the header had to be split.

@hemslo has an implementation example at https://github.com/hemslo/chat-search/blob/90794a6c1e288942893ab7f222c375fbf7612380/app/dependencies/document_transformer.py#L51

# What is Clowder?
## Clowder is a
aaa
## Clowder is b
bbb
...
ccc

The final chunks should be:

# What is Clowder?
## Clowder is a
aaa

and

# What is Clowder?
## Clowder is b
bbb

and

# What is Clowder?
## Clowder is b
ccc