Document what to do with a failing workflow

[zkamvar]

If someone creates a lesson outside of The Carpentries and they do not update their workflows, they might run into a situation like this where a workflow has outdated components and no longer runs:

If this is a toy repository, they should delete it. If this is a repository with lesson content that they need updating, they should add a SANDPAPER_WORKFLOW secret to their repository and run the 02 Maintain: Update Workflow Files action to create a pull request that will update the workflow files.