Add support for MoveIt2 robots

[ycheng517]

What this does

This PR adds support for robots using MoveIt2, the manipulation platform for ROS 2. It opens the door to use LeRobot with many MoveIt2-compatible robot arms, such as ones from Universal Robots, Franka Robotics, KUKA, and more. Example support for the Annin Robotics AR4 is added since that's the arm that I have.

Demo video of keyboard teleoperation with an AR4 arm:

lerobot_demo.mp4

This PR features:

• Flexible config that can be extended to different types of MoveIt2 robots
• a documentation page highlighting how to customize configs for different MoveIt2 robots, and how to run the setup.
• Full tests of the robot class
• I've only implemented velocity control and joint position feedback for now, but can add additional control and observation modes later, if this PR gets adopted.

How it was tested

• Added tests/robots/test_moveit2.py.
• Keyboard teleoperation with the robot arm, as shown in the demo video

How to checkout & try? (for the reviewer)

pytest -sx tests/robots/test_moveit2.py 

Let me know if you'd like to try it with a MoveIt2 arm in simulation!