docs(zones): show how to keep AngularFire calls in an injection context

[armando-navarro]

The zone-wrappers guide explains what the "outside of an Injection context" warning means and how to change its log level, but never shows how to fix the code that triggers it. #3629 asks for exactly that.

This adds a "Keeping calls inside an injection context" section covering the most common trigger: an AngularFire API called inside an async callback (for example a Firestore query built inside a switchMap on the signed-in user), where the callback runs after the synchronous injection context is gone. The recipe captures an EnvironmentInjector while the context is active and re-establishes it inside the callback with runInInjectionContext, plus a note to prefer hoisting the call out of the callback when it doesn't depend on the async value.

Docs-only; no code or behavior change.

Fixes #3629

[tyler-reitz]

Good addition. The two-example structure works well: hoist when you can, runInInjectionContext when you need the per-emission value. The EnvironmentInjector field pattern is the right way to bridge the sync/async boundary.