Multiple libraries in one collection

[TheDreadPirate]

Collections are an option. Have your documentaries in their own shows and movies library, then add all the documentaries into a unified collection.

Another option is to use an external tool to generate NFOs and download images then use the Music Videos library type, which doesn't have external metadata providers by default, and Jellyfin will use your externally generated NFOs.

Tiny Media Manager and Sonaar are both tools that you can use to pull down metadata external to Jellyfin.