Recommend using SI units

[jcornaz]

@bit-garden rightfully explained here why it is important to not use the pixel as a unit, but rather meter* instead.

*: it is not really about "meter", but rather using a unit for simulation so that object sizes are close to 1. So a game that simulates planetary bodies may use Mm, and a game at a microscopic scale may better use µm.

But that is not so intuitive for users who just start with heron, and they can encounter strange behavior (like #276).

So, here is what we should do to improve heron in this area:

• Update documentation to recommend using SI units as well as discourage using pixel as a unit
• Update heron examples to follow that recommendation and show how to use a different unit-scale for the physics than pixels
• Provide a metric-scale resource, so that heron automatically translates the pixel-unit of transforms into meters in the rapier simulation (something inspired by bevy_rapier's solution)

[bit-garden]

Maybe include a way to dynamically update the pixel to meter ratio for screen resizes, like mobile devices.