Move package.xml format 1 to 2 migration to its own page (backport #4778)

[mergify[bot]]

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

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  This is an automatic backport of pull request #4778 done by Mergify.

[github-actions[bot]]

HTML artifacts: https://github.com/ros2/ros2_documentation/actions/runs/11124004356/artifacts/2000227951.

To view the resulting site:

1. Click on the above link to download the artifacts archive
2. Extract it
3. Open html-artifacts-4783/index.html in your favorite browser