Revert "960 Grid System PSD 문서 추"

This reverts commit 635ba6d65b7bef1fa95bb93c51b075a76c74723d.
Revert "Revert "960 Grid System PSD 문서 추""

This reverts commit f53e41ef8274fac774192a2812022cc73860228f.
Grid 컬럼(1, 2, 3, 4단) 추가