Polish the ROS 1 -> 2 CMake instructions (#4767) (#4775)

* Polish the ROS 1 -> 2 CMake instructions

* Use section headings to CMakeLists.txt steps appear in the TOC
* Use modern CMake targets instead of standard CMake variables
* Use Active voice and second person perspective
* Include links to real examples

Signed-off-by: Shane Loretz <[email protected]>
Co-authored-by: Chris Lalancette <[email protected]>
(cherry picked from commit 9d6552c)

Co-authored-by: Shane Loretz <[email protected]>