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.

[BNAndras]

I think configlet uses a locally cached copy of the problem-specifications. That should be accessible so we might be able to use that for test generation as well, removing the need to manually keep a copy of the problem-specs repo.