Challenge questions for level-6

[springcomp]

This pullrequest adds a couple of challenge questions for level-6.
It attempts to give clear examples for:

• A valuable Tempo Clue that gets two new cards to become playable.
• A non-valuable Tempo Clue that Chop Moves the card on the chop.

[Zamiell]

if you click on the red x you can drill down to the build error, which is this:

Exhaustive list of all broken links found:
  - Broken link on source page path = /challenge-questions/level-6:
     -> linking to beginner/minimum-clue-value-principle (resolved as: /challenge-questions/beginner/minimum-clue-value-principle)

[Zamiell]

also btw, you don't have to force push, you can just keep pushing normal commits and they will be squashed