Move package.xml format 1 to 2 migration to its own page

[sloretz]

This moves the guide on migrating package.xml files to format 2 into it's own page. I also made some changes to it.

• Added introduction, prerequisites, and test sections
• Made section headings more descriptive
• Moved <depend> to the bottom to catch simplifications that can be done after the <run_depends> are replaced
• Used .. note:: for the CMakeLists.txt comment
• Use active voice and second person perspective

[github-actions[bot]]

HTML artifacts: https://github.com/ros2/ros2_documentation/actions/runs/11118737215/artifacts/1998919526.

To view the resulting site:

1. Click on the above link to download the artifacts archive
2. Extract it
3. Open html-artifacts-4778/index.html in your favorite browser