Exercise generator fails if no description.md available for exercise

[BNAndras]

bin/lisp_exercise_generator.py tries to generate exercise instructions from a description.md, but not all exercises have that file. This means the script fails early in the process. Some exercises like meetup have an introduction.md and an instructions.md. The script should check for those files as well, or we can also have configlet create the documentation for us. If we do that, configlet create --practice-exercise <slug> would add the exercise to the track config, grab the instructions, and create placeholder files using the files list in the track config. The generator can just overwrite the placeholder files for the test suite and student stub.