Document the new compatibilityFlags query API

[jasnell]

Preparing to land in the internal repo. Should be in production within the next few days

[jasnell]

No, we would need to define it. I think if we defined the type as Record<string, boolean> that would be sufficient and we wouldn't need to define a type for each individual flag. Should defer to folks who are more familiar with how the type definition system works tho.

[KimJ15]

👋Hi there, we’re going to close this PR. All PRs prior to August 12 need to be reworked due to the platform migration to Astro.