Drop execution artifacts that were committed on this branch

foo.py, newfile.txt, numbers.txt, output.txt, output2.txt, test_table.csv
and us_cities.txt are all written by the lectures themselves during
execution (%%file / %%writefile cells in getting_started, python_essentials,
python_advanced_features and debugging), and every one of them is matched
by .gitignore. They were picked up by the initial mystmd setup commit and
do not exist on main.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: mmcky