DEVONthink and DEVONnote already come with a number of plain and smart templates for everyday use. You can find them in the Data > New from Template menu. But you can easily also add your own documents to this menu.
Create your file in DEVONthink or DEVONnote, or import it. The select it and choose File > Export > as Template. The save dialog already points to the “Templates.noindex” folder that holds all templates used by the app. Name your document and save it. You can also create your own sub-folders here.