Copy completed notebooks to training website

[github-actions]

Add completed notebooks to the completed-notebooks directory by manually running the GitHub Action copy-completed-notebooks.yml.
This action requires two input arguments that you will be prompted to provide:

• Which training module is being taught
• The training-modules repository tag from which notebooks should be copied

Optional: You can also remove the .gitkeep file from completed-notebooks.

[sjspielman]

Completed with #17 and #18