Improve collision reaction section

Collision reaction is chaotic, messy and badly explained, it should be restructured.

Put more emphasis on shallow-axis reaction, followed by other, more specific stuff.

Find the collision reaction section in the repository's documentation, likely in a markdown or text file. Review the current structure and content. The goal is to reorganize it to emphasize shallow-axis reaction first, then other specific details. Check for any existing comments or linked resources to understand the intended improvements.

索引モデルが issue の本文から書いたものです。