Add section on package data

[ttimbers]

In the R packages book, they document how to distribute data with a package. This can be done with Python too (e.g., pandas does this). We should document how to do this with Poetry. It is not well documented elsewhere and may fall under the include and exclude docs. Here are two relevant issues discussing this:

• Better documentation for how files are included in built artifacts python-poetry/poetry#2015
• Support for data_files python-poetry/poetry#890