Example for PyPI workflow

[jwnimmer-tri]

All of our examples show complicated C++ things like CMake, Bazel, Ament, etc. The majority of our new users are Python users though!

We should have an example showing the PyPI workflow. If nothing else, a README linking to https://drake.mit.edu/pip.html would be a starting point. We could probably show an example using poetry or something though.

\CC @rcory FYI