build: mkdir demos before building docs. fix #792
build: mkdir demos before building docs. fix #792

greedying/vux,huixt/hxvux,airyland/vux,greedying/vux,szgxwj/vux,huixt/hxvux,airyland/vux,szgxwj/vux,huixt/hxvux,airyland/vux,greedying/vux,szgxwj/vux