Document the whole collision pipeline

[nvtw]

Document the whole collision pipeline

[coderabbitai]

A summary of the changes CodeRabbit can apply:

• Add a new summary document at docs/concepts/collision_pipeline_detailed.md that consolidates the full collision pipeline (overview of CollisionPipeline vs CollisionPipelineUnified, five-stage workflow from AABB to contact writing, broad/narrow phase algorithms including three broad-phase modes and three mesh paths, collision filtering, performance notes, API examples, tests, code references with line numbers) and records key findings (file/line counts, kernel sizes, multi-contact behavior, and optimization guidance).

• Add a new documentation file docs/concepts/collision_pipeline_detailed.md providing a detailed technical overview of Newton’s collision pipeline (stages: AABB, Broad Phase with NxN/SAP/EXPLICIT, Narrow Phase with GJK/MPR, contact manifold and writing), implementation notes (kernel and module references), performance numbers, usage examples, and references.

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[nvtw]

Mention tuning parameters like max_triangle_pairs: int = 1000000