Add collision handlers tutorial

[viblo-majority]

Summary

• Add a new tutorial explaining how collision handlers work in Pymunk
• Cover the four collision phases (begin, pre_solve, post_solve, separate)
• Include practical examples: breakout-style brick destruction, one-way platforms, and collision damage calculations
• Document the Arbiter API and common pitfalls

Test plan

• Build documentation locally with cd docs/src && make html
• Verify tutorial renders correctly
• Test example code snippets work as expected

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