Game logic chapter

[alice-i-cecile]

If you'd like to read (or review) this PR, you'll need to:

1. Install zola.
2. Clone this repo.
3. Use zola serve --open from that directory.

With #182 in a reasonable state, it's time to dive into the next chapter! I'm opening this early, so others can watch, learn and provide feedback as I work.

Contents:

• Introduction
• Scheduling systems
  • Ambiguous systems example
  • Equivalence of data access models
• Custom game loops
• Run criteria
• States
• Events
• Time and timers
• Async tasks

[alice-i-cecile]

As I discussed with @cart on Discord, my explanation of data access compatibility doesn't match the current implementation. However, I believe they're formally equivalent, and the current explanation nicely unifies the incompatibility checks done by system parameters and ambiguities (what I'm calling hypothetical incompatibility) with the checks done by the scheduler (factual incompatibility).

This framing is also, IME, very useful for reasoning about parallelizability of complex data access patterns.

DIscussion link for the equivalency: bevyengine/bevy#3119

[cart]

Closing as we will very likely start new PRs (and use this as a reference) when we pick up the new book effort again.