Resolve "Create a CLL"

Added an exercise for transforming (🤖 ) a singly linked list into a circular linked list, works the in a similar fashion as !34 (merged).

@MrHug , you mentioned in #25 (closed) that you would like to provide the "motivation" for using a cll? I currently have a random made up story as description, feel free to change it. It's not necessarily the best example for using a cll. 😛

Closes #25 (closed)