gather example projects in a clear view

One of the purposes of DO is to learn about DA/DQ.
For DA, several examples have been suggested in the literature and only 2 of them are currently implemented (The oil Wildcatter and the Used car buyer problems). Others may be interested too, and we may foresee an increase of such cases.

So 2 features can be implemented:
1. Examples projects could be read and copy only. A user who wants to modify such a project would need to first duplicate it.
2. On the project page, we would need to be able to select between example and work projects

Start by tracing how the project page lists and opens the two existing examples, The oil Wildcatter and the Used car buyer. Review the current project and duplication flows; done means examples are read-only, users can select examples separately from work projects, and modifying an example requires duplication.

Written by the indexing model from the issue text.