Tutorial instructions turn src into a git repo

[alex-quin-gabriel]

See https://plansys2.github.io/tutorials/docs/simple_example.html#overview

git clone -b <ros2-distro>-devel https://github.com/IntelligentRoboticsLabs/ros2_planning_system_examples.git src

This seems undesirable and unnecessary.
Could we just have a ws/src/ros2_planning_system_examples folder structure? That would make it easier to combine the tutorial code with people's own experiments.

Also affects the other tutorials.