party-robot: update introduction #1728
Description
The party-robot exercise unlocks the packages concept, but its introduction.md document does not explain what packages are (it does show how they can be used for the fmt package).
Firstly, the introduction.md document of the party-robot exercise should be updated to better explain what packages are. There is already content for this in the introduction.md document of the packages concept.
Secondly, the packages concept should mention how public/private visibility works. See #1720 for sample text for this. The public/private visibility explanation should be added to the exercise's introduction.md and the concept's introduction.md document.