Added retrieved documents in QuestionAnswering

[ezimuel]

This PR adds the possibility to return the retrieved documents in QuestionAnswering using the similarity search in searchDocumentAndCreateSystemMessage(). This can be useful when implementing RAG architectures to retrieve the documents (chunk) used to generate the answer.

I proposed to add a QuestionAnswering::getRetrievedDocuments() that returns the array of Documents retrieved from the similarity search, as follows:

$qa = new QuestionAnswering($vectorStore, $embedding, $chat);

$result = $qa->answerQuestion('What is the capital city of Italy?');
print($result); // The capital city of Italy is Rome

$docs = $qa->getRetrievedDocuments();
printf("Answer generated using the following contents:\n");
foreach($docs as $d) {
    printf("%s\n", $d->content);
    printf("Source: %s\n", $d->sourceName);
}