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

* 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 <shane.loretz@gmail.com>
Co-authored-by: Chris Lalancette <clalancette@gmail.com>
(cherry picked from commit 9d6552c)

# Conflicts:
#	source/How-To-Guides/Migrating-from-ROS1/Migrating-CPP-Packages.rst