feat: add configurable furniture_mode options to markdown serializer

[riyo264]

Description

Context

Fixes #665

Following up on the architectural discussion during the MS Word backend update in the main docling repository, this PR moves the configuration and handling of document furniture (headers and footers) to the serialization layer in docling-core.

Currently, items on the FURNITURE layer are omitted by default during Markdown serialization to prevent repetitive page layout clutter. This PR introduces a configurable parameter to let users selectively include headers and footers, which is highly beneficial for documents where margin metadata (like document versions, classification markers, or template notices) carries crucial context.

Changes Introduced

• Added FurnitureMode Enum: Defined three core serialization modes:
  • none (Default): Retains existing behavior by dropping all furniture elements.
  • all: Renders every header/footer sequentially as they appear across page boundaries.
  • distinct: Implements text-based deduplication, serializing each unique header/footer exactly once when first encountered.
• Updated MarkdownParams: Added the furniture_mode field and introduced a Pydantic @model_validator(mode="after") to dynamically inject ContentLayer.FURNITURE into the active processing layers whenever a rendering mode is active.
• Intercepted MarkdownDocSerializer.serialize: Built a stateful tracking mechanism (self._seen_furniture) directly onto the document serializer instance to handle the distinct text-matching logic cleanly during the tree-walk traversal.
• Test Coverage: Added a comprehensive unit test suite in test/test_serialization.py (test_md_furniture_modes) verifying the behavioral purity of all three configuration modes.

Verification & Testing

Ran the serialization test suite locally using Python 3.14:

pytest test/test_serialization.py

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