Add example for migrating a Python package from ROS 1 to 2

[sloretz]

This adds a guide walking someone through migrating a ROS 1 Python package to ROS 2. I assumed the reader is familiar with ROS 1 and Python, but is not familiar with ROS 2's Python APIs or best practices.

[github-actions[bot]]

HTML artifacts: https://github.com/ros2/ros2_documentation/actions/runs/11353745523/artifacts/2060170324.

To view the resulting site:

1. Click on the above link to download the artifacts archive
2. Extract it
3. Open html-artifacts-4780/index.html in your favorite browser

[kscottz]

This is a great start, what I would recommend we do this in a step by step fashion. For example, first show the package.xml upgrade process, then move on the next file. This approach would keep the before and after files right next to each other.

[sloretz]

@kscottz @clalancette this guide is ready for review!

[sloretz]

@clalancette @kscottz all feedback addressed. PTAL :bow: