Mail Merge is an AppleScript replacement for the mail merge feature that was omitted from Pages 5.

Create a Numbers document with one sheet, containing one table, with one header row and no header columns. Fill with data. Create a Pages document (if it has body text, almost certainly ending in a page break), and write "%Column Name%" to reference data in the column from your Numbers table whose first (header) cell is "Column Name". Open both documents. Run script. Rejoice!