docling-core: export DoclingDocument to markdown/text locally (Python parity)

DoclingDocument in Java deserializes from JSON, but there is no local export API (`export_to_markdown()` / `text`) like in Python.

We persist Docling JSON as canonical artifact and need text/md in later (async) document processing stages (moderation, etc.) without re-parsing the original file and without a Serve round-trip (JSON_DOCLING → md|text works but adds HTTP coupling).

Proposal: add local export in docling-core, e.g. `toMarkdown()` / `toText()`.

Use case: offline/async workers reading archived JSON only.