feat(markdown): add footnote serialization support

[ShrillHarrier]

This PR is related to the Improved Footnote Serialization in MarkdownDocSerializer.

It is a Feature Request submitted by simonschoe in docling-project.

The features added include serializing a footnote in the form [^{Identifier}]: {Description}. This is done for Table and Picture items, as footnotes are linked to those.

In general, footnotes in .md files should look like:

[^5]: https://github.com/tesseract-ocr/tesseract
[^6]: https://github.com/VikParuchuri/surya
[^7]: https://github.com/lukas-blecher/LaTeX-OCR

Tests Added:

• test_table_with_footnotes_markdown()
• test_picture_with_footnotes_markdown()
• test_table_export_to_markdown_with_footnotes()

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Related Documentation

1 document(s) may need updating based on files changed in this PR:

Docling

What are the differences between vlm_pipeline_model_local and picture_description_local in Docling, and how do image descriptions, OCR, and table extraction work together? Also, how do the include_annotations and mark_annotations properties affect exported output?

View Suggested Changes

@@ -67,6 +67,9 @@
 - `compact_tables` (bool): Whether to use compact table format without column padding (default: `False`, Markdown only)
 - `traverse_pictures` (bool): Whether to traverse into picture items and serialize their text children (default: `False`)
 
+**Footnote Serialization in Markdown:**
+When tables or pictures have associated footnotes in the document, these footnotes are automatically serialized in the markdown output using standard markdown footnote syntax: `[^{Identifier}]: {Description}`. The identifier is extracted from the first part of the footnote text, and the remaining text becomes the footnote description. This formatting ensures that footnotes attached to Table and Picture items appear correctly in the exported markdown.
+
 **Handling OCR Text in Scanned/Image-Based PDFs:**
 When processing scanned or image-based PDFs with `force_full_page_ocr=True`, the layout model classifies full-page scans as `PictureItem` nodes. OCR text items are added as children of that picture node in the document tree.
 

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