Improve example "create model" notebook or add another

We need an example to create a model from scratch like happening in https://github.com/bioimage-io/core-bioimage-io-python/blob/main/presentations/create_ambitious_sloth.ipynb

but for a simple 2D Unet.

https://github.com/bioimage-io/spec-bioimage-io/blob/main/example/load_model_and_create_your_own.ipynb falls short as it first loads an existing model.

cc @bioimage-io/spec-dev