Introduce insert-license pre-commit hook, Switch `#` comment instead of `"""` docstring for the copyright and license statements
This can cause some issues in the future.
### Todos
- [x] Introduce [`insert-license` hook](https://github.com/Lucas-C/pre-commit-hooks#insert-license)
- [x] Update license header without docstring but with `#`, thanks to [`insert-license`](https://github.com/Lucas-C/pre-commit-hooks#removing-old-license-and-replacing-it-with-a-new-one).
- [x] Get rid of `update_copyright_year.sh` which becomes useless
- [x] Document how to update the copyright year
issue