docs: add visual-safe reader export example

[one-num-three]

Summary

Adds a documented export_visual_blocks.py example for converting research PDFs into reader-oriented Markdown with source-positioned visual assets.

Why

A semantic PDF conversion can emit chart labels, table cells, and diagram glyphs as ordinary text. In a reader this creates duplicated content, broken reading order, and apparent gibberish after a figure. Vector-only charts can also be absent from image extraction.

What changes

• preserve Docling pictures and tables as Markdown image assets;
• crop formula provenance boxes from the source PDF for faithful rendering;
• skip text items whose bounding box is substantially covered by a visual item, while retaining captions;
• detect captioned vector/raster figure and table regions from PDF geometry as a fallback;
• add the rendered example to the documentation navigation.

Impact

This is an opt-in documentation example. It does not change Docling's conversion defaults or public APIs. It needs the optional pymupdf package.

Validation

• ruff check docs/examples/export_visual_blocks.py
• ruff format --check docs/examples/export_visual_blocks.py
• python -m py_compile docs/examples/export_visual_blocks.py
• verified the required PdfPipelineOptions fields against an installed Docling environment
• the example CLI loads and exposes --input, --output, and --pages

A real-PDF end-to-end smoke run was attempted with a one-minute local command limit; it exceeded that limit during Docling model initialization, so full conversion should be exercised in the project CI or a fully warmed environment.

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[one-num-three]

    你好，谢谢你给我写信,现在我不在电脑旁,在时我会回信给你.

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[dolfim-ibm]

@one-num-three can you please provide an example of the export with and without your code?