add a merge function for DoclingDocuments

When parsing multiple pages separately, I would like to merge them all into one document.
A generalization of this idea is to be able to merge arbitrary sets of Docling documents.

Could either be as helper:
```
helper.merge_docs([doc1: DoclingDoc, doc2:DoclingDoc,...]) -> DoclingDoc
```

or as method returning success as bool:
```
doc1.merge_into_doc([doc2,...]) -> bool
```