Overhaul of the formation
Essentially:
- All notebooks have been read and more or less amended.
- The notebook about inheritance has been heavily amended.
- Some passing remarks on C++ 20 have been added here and there.
- The copyright year has been updated.
What has NOT been reviewed this time are the TPs (except regarding the copyright years).
One of the reviewers (@steff or @vrouvrea) should also handle this (very quick) issue