Restructure example/petab_import.ipynb

[dweindl]

https://pypesto.readthedocs.io/en/latest/example/petab_import.html

Currently:

[confuse the user with many lines of code] 
[say you could have also done it in just 2 lines]

Preferred:

[show petab import in 2 lines] 
["if you need more flexibility, e.g. to do X, this is how you can do it"]