[Feature Request] Federation: direct sharing of documents over trusted & connected instances

Description Current state While with Groups and Users you have a control over who can see what and do actions, hand-over a document by the sharing process is kind of cumbersome, especially if you have two persons with each one running a paperless-ngx instance. The already set metadata gets lost and one person has to export/create a shareable link of the document and the other has to download it. Case Each one of multiple persons (like family, friends) run a separate paperless-ngx instance for private document archiving. Some may run two instances for storing private and business documents one one machine. Over time, often invoices or some other document types have a correspondent of a friend or family member because of different circumstances. These documents may belong primarily to them, but in context also to you. The process of putting these documents to the right place could be made more easy. Fu(ea)ture suggestion Two or more paperless-ngx instances can connect and …