Drag another document into the body of your RTF document while holding Option and Command This ⌥⌘Drag action will make the curvy arrow appear that indicates you are dropping a link to the other document into the body of the document you are working on. (If you merely drag another document into the body of the RTF without the modifier keys you’ll end up dragging the contents of that other document into the body of the document you are working on.)
Type the name of the other document – if the other document is in the same databasee. If you have Preferences > Editing > Wikilinks set to Automatic and Names and Aliases, then a link will be created. If you have more than one document with the same name then the results are hard to predict (they follow a rule but it’s not easy to figure it out manually).
Select the document you want to link to and copy its item link using the Edit menu or the contextual menu. Paste that link into the body of the RTF document – DEVONthink will display the name instead of the URL, but it is a clickable link.