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:

```python
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!!)