Did you try something like this?
If you replace the content of the “Exec:” line with, for example, “nautilus Documents” (or “nemo Documents”), or a further subfolder within the Documents folder (“nautilus Documents/Subfolder”, that will get you there:
Create MyFolder.desktop and MyIcon.png as described in the link. The contents of the .desktop file should be (for example):
[Desktop Entry]
Name=MyFolder
Exec=nautilus Documents/Subfolder
StartupNotify=true
Terminal=false
Type=Application
Categories=Utilities;
Icon=/home/purism/.local/share/icons/MyIcon.png
It gets added to your app grid. It’s not possible to use the “Desktop” directory on the L5, as far as I know. (Or rather, you can put files or directories there from within the file browser, but they don’t display on a “desktop” screen. The “desktop” in the file structure is not the app grid/home screen; it’s the blank space/background that appears when you first launch an app.)