Revert my previous change to restore the original collision group example

[Lucatacos]

Changes

Reverting my change (#984) because the original example is clearer and handles all cases better. It properly sets collision groups for parts added after the character spawns, which my version didn’t cover. This makes the official documentation more complete and easier to understand.

Regarding performance, the difference is minimal for most use cases, but the original version is safer as it handles dynamic parts correctly without impacting performance noticeably

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