Setup Kobuki model for Gazebo and Nav2 Simulation

[j3soon]

Extend the getting started guide of Nav2 by incorporating a custom robot model (Kobuki). Note that not only the visualization of the robot need to be changed, the motion of the robot also need to follow the kinematic model of the custom robot.

• Extract the kobuki model
  • http://wiki.ros.org/kobuki/Tutorials/Gazebo%20Simulation
  • https://github.com/yujinrobot/kobuki/tree/melodic/kobuki_description and maybe:
  • https://github.com/yujinrobot/kobuki_desktop
  • https://github.com/yujinrobot/kobuki_core
• Integrate the model into Nav2 & Update README
  • https://navigation.ros.org/setup_guides/index.html