feat: enable heading-only chunks for empty-section headings

[vagenas]

Adding an option to address cases (e.g. related to docling-project/docling#287) like:

## Section 1

## Section 1.1

## Section 1.1.1

Foo

where currently Section 1 and Section 1.1 are not emitted in any chunk, as they are considered "shadowed" by fellow-level2 Section 1.1.1.

[github-actions]

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[dosubot]

Related Documentation

Checked 8 published document(s) in 1 knowledge base(s). No updates required.

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[dolfim-ibm]

lgtm