[Feature Request] Bulk action "Link" to link the selected documents to each other

Description Add an Actions > Link bulk operation that links the selected documents to each other. Today, linking related documents in bulk is possible only through Custom fields, which works as a *value assignment*: every selected document receives the same value. For a document link field that value is itself a document, which has to be found by hand in a picker. That shape does not match the intent. When several documents belong together, there is no natural "centre" to point at, and picking one arbitrarily produces a star rather than a group. Concrete case: a supplier answers an order with one email carrying an order confirmation and two approval drawings. These arrive as three separate documents. They belong together, and none of them is the obvious centre. Proposal. Add Link to the bulk Actions menu, next to Merge and Merge as versions: 1. Select the documents that belong together 2. Actions > Link 3. Pick which document link field to use, if more than one e…