Improve, restructure documentation and add screenshot

docs/debian_installer/setup_debian_4_install_guest_additions.sh

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+# Mount Guest Additions and run the installer
+echo "Mount Guest Additions and run the installer..."
+sudo mount /dev/sr0 /media/cdrom
+cd /media/cdrom
+sudo sh ./VBoxLinuxAdditions.run
+echo "Done"
+echo "Please follow the instructions:"
+echo "Create Shared Folder. VM: Devices=>VM: Devices=>Shared Folders=>Shared Folders Settings"
+echo "Name: codeocean, Path: path to your local codeocaen repository on the host machine."
+