Updates to the SentenceWindowRetriever

[sjrl]

Is your feature request related to a problem? Please describe.
We are finding that the SentenceWindowRetriever is a powerful RAG tool for some client projects. We are especially testing it out in with some early prospects. While using it @JasperLS and I identified some aspects that could make it easier to use.

Describe the solution you'd like

• Ensure that the documents in context_documents are sorted by split_idx_start. We noticed that the documents are only sorted when merging them into one text blob in the merge_documents_text function.
• Would it be possible to also export a list of merged documents. So have an output with type List[Document]. This would make it easier to use in a downstream prompt builder since we typically want to also use the metadata of the resulting merged Document.

I realize for the second request it is possible to use a nested loop to overcome this in a Prompt so I wouldn't say it's a hard requirement, but it would be more convenient than working with a List[List[Documents]]