Support multiple InvoiceReferencedDocument elements for BG-3 according to ZUGFeRD version 2.3

[HoffmannTom]

The ZUGFeRD version 2.3 extended the <InvoiceReferencedDocument> element cardinality from 0..1 to 0..n.
This enables the reference of multiple invoice documents. For a final invoice, all preceding partial invoices must be referenced.

Currently, only one invoice document can be referenced which is not sufficient for use cases, where more than one partial invoice was sent out.

A similar issue #536 was already created but with a different purpose.

It should be possible to add multiple invoice references to one invoice and thus create multiple InvoiceReferencedDocument elements, e. g.:

            <ram:InvoiceReferencedDocument>
                <ram:IssuerAssignedID>INV109</ram:IssuerAssignedID>
                <ram:FormattedIssueDateTime>
                    <qdt:DateTimeString format="102">20241210</qdt:DateTimeString>
                </ram:FormattedIssueDateTime>
            </ram:InvoiceReferencedDocument>