Add image_dir parameter to export_to_markdown for automatic image saving

Problem

When using ImageRefMode.REFERENCED, users must manually:

1. Iterate through all picture elements
2. Save each image to disk
3. Update the URI for each element
4. Then call export_to_markdown()

Proposed Solution

Add optional image_dir and image_path_prefix parameters to export methods:

markdown = doc.export_to_markdown(
    image_mode=ImageRefMode.REFERENCED,
    image_dir=Path("./output/images"),
    image_path_prefix="images/"  # relative path in markdown output
)

When image_dir is provided:

• Automatically save images to that directory
• Generate sequential filenames (e.g. image_001.png)
• Use image_path_prefix in the markdown output

Affected Methods

• DoclingDocument.export_to_markdown()
• PictureItem.export_to_markdown()
• Potentially HTML export methods too

Would this be an interesting feature to have? Happy to submit a PR.

(PS: Keep up the great work!!)