Additional Reference on PEPPOL Invoices

Add support for an extra reference number on PEPPOL invoices, such as a SWIFT Service Entry Sheet (SES) number. This would be implemented via a Contract Document Reference field in the PEPPOL UBL invoice, allowing invoices to be linked to contracts or service approvals in external systems. Benefit: Easier invoice matching, fewer rejections, and better integration with procurement and ERP workflows.