footer固定したい
positionをabsoluteや、width:100%で試した
https://techacademy.jp/magazine/19410
cssのボーダー折りたい
https://stackoverflow.com/questions/7059597/slanted-corner-on-css-box
https://stackoverflow.com/questions/7324722/cut-corners-using-css
https://www.w3schools.com/cssref/css3_pr_border-top-right-radius.asp
https://www.w3schools.com/cssref/tryit.asp?filename=trycss3_border-top-right-radius
https://stackoverflow.com/questions/31785198/css3-diagonal-border-line