Documentation improvements for basic setup and scheduling

[crvarner]

I'm new to bevy_xpbd and I'm really struggling with what feels like basic "getting started" type things. In particular I don't know on which schedules I should be registering my systems and PhysicsPlugins.

Conventional game design wisdom says I should handle input on Update and do my physics simulations on FixedUpdate. This plugin appears to instead create a bunch of additional schedules hanging off PostUpdate (why not FixedUpdate?). I know I can override which schedule to run the physics on and I've read the documentation describing the different schedules defined internally, but frankly I'm struggling. This is further complicated when using Kinematic RigidBody, since we need to implement their physics systems manually. It isn't clear which schedules those should run on in order to play nicely with the physics.

I feel like input on Update + physics on FixedUpdate + Kinematic character controller is a very common pattern for people starting out on a game. I think it would be helpful to either improve the documentation or flesh out the basic_kinematic_characters.rs example. In particular, it would be great to explain why we are doing certain things and relate them to common game design best practices.