and change all the challenges to new `md` format.
* fix: remove isHidden key from tool template * fix: removed isHidden key from English challenges * fix: remove isHidden key from Chinese challenges
* fix: renamed basic html cat photo app steps * fix: renamed css-variables project steps * fix: renamed d3-dashboard filenames * fix: renamed rpg-game filenames * fix: renamed functional-progamming-spreadsheet filenames * fix: renamed calorie-counter project filenames