Document Bevy `Transform` interpolation

[Jondolf]

What problem does this solve or what need does it fill?

Related to #95.

The movement of rigid bodies in the Bevy plugin can be interpolated by using TimeStepMode::Interpolated and adding the TransformInterpolation component to a rigid body. However, this is not documented anywhere on the website.

What solution would you like?

Document transform interpolation, how it works, and the potential tradeoffs it has. This could be in the same section as #95.