Server Side Includes (SSI)

With the item selected, click the first tab in the Inspector palette and change the Type popup menu from HTML to URL. Type the file’s name in the text field, including the path to the folder as necessary, and change Content from Image to HTML. That’s virtually actually all there is to this: if you just want the client to work with plain text, they can use any text editor they like to alter the included file and then upload the fresh copy to the separate folder.