exercise_en.ipynb: Some updates

[MMesgar]

Thanks for providing such an informative material.

Some points to update

1. clear the output of the cell. Students should run the cells and see the outputs themselves.
2. remind the students at the beginning to upload the file in Google Colab.
3. define Tasks or python exercises.
4. "Congratulations! You are now done with the first exercise of GKI." is not consistent with the template.