Add hints towards the "about" version of a concept

Currently, there is no indication for a student that the concept page will provide more detailed information once the concept is unlocked. The student can only find out if they click on the concept again later by coincidence.

Some ideas:
* A note like "more detailed information will become available once you unlock this concept" on the "introduction" version of the concept page
* Some explanation + link to the (now detailed) concepts after completing the respective concept exercise, e.g. in this popup