Polish the ROS 1 -> 2 CMake instructions

[sloretz]

This updates the CMake portion of the ROS 1 to ROS 2 guide, and fixes one bug in the C++ code

• Included links to real examples in the core (Mostly tf2)
• Updated section headings to CMakeLists.txt steps appear in the ToC
• Used modern CMake targets instead of standard CMake variables
• Used Active voice and second person perspective
• Removed unnecessary code from example code in Unit tests section
• Make the example package.xml start with format 2 since that's different from migrating from ROS 1 to ROS 2.

C++ bug fix: use ->publish( instead of .publish(

I have tried out all steps of the example part of the guide to make sure they work.

[github-actions[bot]]

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