Challenges

 - Implemented CSS for challenges (same as for Objectives, just different color)
 - removed redundant CSS for Objectives / Key Points
 - Clarified & rearranged challenge in lesson 02.
 - added contrib instructions.
Each challenge is supposed to have a meaningful title such as "Find Distinct Sites" (to make it easier to find) and then the style `{.challenge}` so that it can be detected and reformatted by Pandoc.