[Feature Request] Add grouping options to the document view

Description Problem / Current Behavior Paperless-ngx already provides flexible options for filtering and sorting documents. For example, documents can be filtered by: • tags • correspondent • document type • custom fields • other metadata The resulting document list can also be sorted by a selected field in ascending or descending order. What is currently missing is an additional grouping option. Sorting changes the order of documents, but it does not create visually separated groups based on a selected metadata field. Desired Behavior I would like to be able to group the current document view by a selected field. For example: • Group by document type • Group by correspondent • Group by tag • Group by storage path • potentially group by selected custom fields The grouping should work independently from filtering and sorting. This would allow a user to: 1. filter the document set…