docs: add robot scaling examples for issue #166

[Parsa-Meshkini]

Summary

Closes #166

Adds documentation and runnable examples showing how to scale a robot model from meters (URDF/ROS default) to millimeters.

New files:

• docs/examples/02_description_models/04_scaling_robot.rst — documentation page covering the scaling workflow
• docs/examples/02_description_models/files/04_scaling_robot.py — standalone example using RobotLibrary (no ROS required)
• docs/examples/02_description_models/files/04_scaling_robot_from_ros.py — example loading from ROS and scaling after setting the scene object

What the docs explain:

• scale() applies an absolute (non-cumulative) factor — calling it twice with 1000 stays at 1000x
• scale() can be called before or after assigning scene_object; both work correctly
• Affects all planning I/O (targets, trajectories) automatically

Test plan

• 04_scaling_robot.py runs without errors (PYTHONPATH=src venv/bin/python docs/examples/02_description_models/files/04_scaling_robot.py)
• Output confirms correct scale factor at each step
• ROS example requires a live ROS backend (consistent with other *_from_ros.py examples in this folder)