docs: Point users to `NarrowPhaseConfig::prediction_distance`

[johanhelsing]

Objective

I tripped up on collisions, because I considered the units as meters, but the default value for prediction distance is 1 (presumably pixels)

Solution

Most of the documentation links to the ContactReportingPlugin when talking about contacts.

So link from there to NarrowPhaseConfig::prediction_distance, so they can follow the trail.