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.

首先阅读 DoclingDocument JSON 模型，并将所请求的本地导出行为与 Python 的 markdown 和 text 导出进行比较。在不重新解析原始文件且不使用 Serve 往返的情况下，实现并验证从已归档的 Docling JSON 输出 markdown/text；当两种本地导出形式都可通过提议的 API 使用时，即表示完成。

由索引模型根据 Issue 内容生成。