Create example notebook from presentation (notebook) "Create model from scratch" 

[This presentation](https://github.com/bioimage-io/core-bioimage-io-python/tree/main/presentations) [slides](https://bioimage-io.github.io/core-bioimage-io-python/presentations/create_ambitious_sloth.slides.html) are a good start for another example on how to create a model description from scratch.

