Document timesteps and scheduling for the Bevy plugin

[Jondolf]

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

Related to #91.

Timesteps and scheduling details can be very important for Bevy games. The default timestep in bevy_rapier is a variable timestep, but for a lot of games it can be better to run physics in FixedUpdate with a fixed timestep, or even in a custom schedule for e.g. networking purposes. I believe a fixed timestep is even the default in most game engines like Unity.

It should be properly documented how users can configure their timesteps and the schedule in which physics is run, and what tradeoffs and benefits each option has (e.g. determinism, performance, visual smoothness, and so on).

What solution would you like?

Document timesteps and physics scheduling for Bevy. This could have its own page or be a part of something like #91.