Expose embedded image assets in markdown output

Context

When processing DOCX files, to_document() correctly extracts embedded images into doc.assets (with raw bytes, media type, and origin part). However, these images don't appear in the markdown output from to_markdown_bytes() — there are no ![alt](...) references, and the inline block model doesn't surface image inlines.

Current behavior

doc = anydoc.to_document(docx_bytes)
print(len(doc.assets))  # 1 (608KB PNG found)

md = anydoc.to_markdown_bytes(docx_bytes)
print("![" in md)  # False (no image references)

The asset data is extracted and available, but the markdown doesn't reference it.

Request

It would be useful if to_markdown_bytes() emitted image references (e.g., ![alt text](asset:0) or ![](embedded:image0.png)) at the positions where images appear in the document. The actual bytes are already in doc.assets — consumers could then decide whether to inline them as data URIs, upload them, or send them to a vision model for descriptions.

This would also apply to XLSX and PPTX files that contain embedded images/charts.

Workaround

Using to_document() to get the asset bytes and processing them separately works, though without positional information it's hard to associate an image with the surrounding content.