- drop 'chapter_' prefix for chapter folders
- drop numbers for ToC sub-chapters
- split exercises into one problem set per notebook
- adjust titles in notebooks => show sub-chapter titles on the same
line as the actual chapter titles
- the current video lectures regard the old one-notebook-per-chapter
structure of the project
- as the new structure involves several smaller notebooks per chapter,
there is no one-on-one correspondence to the YouTube lectures any more
=> only reference the YouTube playlist once in the README.md