feat(examples): add robot arm notebook and refine existing examples

[Littie28]

Summary

This PR extends the example notebook series by adding Example 03 (robot arm kinematics) and improving the didactic progression across all three examples.

Changes

New: Example 03 - Simple Robot Arm Kinematics

• Demonstrates multi-level frame hierarchies (5 frames deep: root → base → joint1 → joint2 → joint3 → hand)
• Shows dynamic transformations with animation (rotating joints over multiple steps)
• Introduces data structures with frames (RobotArm class managing multiple coordinate frames)
• Includes point tracking through kinematic chains (tool tip transformation example)
• Adds coordinate system visualization with plot_gismo() helper

Improvements to Examples 01 & 02

• Example 01: Simplified frame creation pattern for better consistency, clarified preferred frame.point() / frame.vector() API
• Example 02: Added explicit connection to Example 01 concepts, enhanced Key Takeaways with progression explanation
• Both: Improved learning flow and consistency across all examples

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.