do not make instructors/instructor-notes.md mandatory

[zkamvar]

The instructor-notes.md file should not be mandatory. We can create it if needed if the authors choose to only write inline instructor notes.

is the instructors/instructor-notes.md file mandatory?

At the moment, consider it mandatory, but thinking about it: it does not need to be mandatory. I can create the file if it does not exist.

_Originally posted by @zkamvar in https://github.com/carpentries/workbench/issues/16