update chapter status legend, and add exercise-in-progress icon to ToC for ch. 1.1
update chapter status legend, and add exercise-in-progress icon to ToC for ch. 1.1

LispTO/llthw,tang-junjie/llthw,LispTO/llthw,LispTO/llthw,tang-junjie/llthw,tang-junjie/llthw