Rework module structure, and improve documentation and Rotation

[Jondolf]

Objective

Note: This PR targets the avian branch instead of main. This is intended, as this is a part of an ongoing rework.

The current module structure is not ideal.

• components just contains a bunch of components with no clear separation of concerns, and some components are still stored elsewhere, so it's not even comprehensive at all.
• resources has the same issue; no separation of concerns, and it's pretty random which resources are there vs. elsewhere.
• Why is constraints a top-level module? It's mainly a solver implementation detail.
• plugins is weird, because it has sub-folders with sub-plugins, and really it's just a folder for basically all functionality in the engine.

The lack of separation of concerns harms the modularity of the engine and can make it harder to find things. For a project at this scale, modules like components are bad.

Solution

• Yeet plugins, components, and resources, flatten the module structure.
• Organize everything by logical concerns.
  • Components and resources related to colliders and collision detection are in collision.
  • Components and resources related to rigid bodies, gravity, and so on are in dynamics
  • Resources related to scheduling are in schedule.
• Add dynamics module for encapsulating rigid body dynamics (solver, sleeping, rigid body components, etc.), which also gives a nice place for more high-level docs.

This PR also contains some other miscallaneous improvements:

• Split the PhysicsSetupPlugin into PhysicsSchedulePlugin and PhysicsTypeRegistrationPlugin
• Fixed broken doc links.
• Generally improved some docs.
• Improved the Rotation type, making it closer match Bevy's new Rot2.
  • This is very unrelated, but it's a part of the ongoing rework anyway and annoying to split out :P

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Migration Guide

• The internal module structure has changed significantly, and types have moved around. Most imports from the prelude should work like before, but explicit import paths may be broken.
• The PhysicsSetupPlugin has been split into PhysicsSchedulePlugin and PhysicsTypeRegistrationPlugin.