Feature Request: Relative paths for images in exported markdown

### Requested feature

Use relative paths for images in exported markdown. Currently, the complete terminal file path is used, which makes portability difficult. This could be selected via a CLI option.

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### Alternatives

Manual editing of exported md files—doable, but irritating if working with many files. Could be automated with a script, but including the option in Docling would be preferable.

Locate the markdown export path and the existing CLI option handling; the issue does not name specific files or tests. Confirm how image paths are currently built, then verify that an option produces relative paths while the existing terminal-path behavior remains unchanged.

Written by the indexing model from the issue text.