carefully motivate init and describe what goes there

I was thinking an example of how imports can change with the init, like, reducing the levels of the imports eg how you can use the package.module once you have init or even package.function() if the depending on the contents of the init file? In pilots of this workshop (and in working with students in research ) they tend to struggle with what to put in the init file, other than pattern matching what's already there.

_Originally posted by @brownsarahm in https://github.com/carpentries-incubator/python-packaging-publishing/pull/71#discussion_r595482503_