[Feature Request] Document Bundles

Description Currently, in Paperless-ngx, documents are stored as individual items and users rely on tags/metadata to associate files that are part of the same real-world document package. I was wondering if it would be possible to introduce a new model for strongly related documents, where multiple files could be grouped into a single document entity. For example: • contract.pdf • appendix.pdf • sales.xlsx • signatures.pdf could be represented and viewed as one document package, such as: "Google Partnership Initialization" rather than requiring users to create tags and manually correlate separate documents. This would be useful for cases where multiple files together represent one business object, such as contracts, applications, compliance packages, or project documentation. The existing document model and tagging system would still be useful for unrelated standalone documents; this would provide an additional way to represen…