Add support for Reachy2

[aliberts]

What this does

Adds native support for the Reachy 2 robot.

• Adds ReachyRobot and ReachyCamera with their associated config.
• Adds examples/12_use_reachy2.md
• Add documentation

How it was tested

TODO

How to checkout & try? (for the reviewer)

• Teleoperation:

python lerobot/scripts/control_robot.py teleoperate \
    --robot-path lerobot/configs/robot/reachy2.yaml

• Dataset recording:

python lerobot/scripts/control_robot.py record \
    --robot-path lerobot/configs/robot/reachy2.yaml \
    --fps 15 \
    --repo-id ${HF_USER}/reachy_wave \
    --tags reachy \
    --warmup-time-s 2 \
    --episode-time-s 60 \
    --reset-time-s 60 \
    --num-episodes 50 \
    --push-to-hub 0 \
    --single-task "When you see someone, look at them and wave."