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