docs: the App Check guide does not cover server-side rendering #3774

docs/app-check.md has no guidance for server-rendered apps.

What I found

• On AngularFire 21.0.0-rc.1 and earlier, following the guide in a server-rendered app fails with ReferenceError: document is not defined (Using with Angular 17 SSR -> document is not defined on initializeAppCheck #3488, AppCheck Build Error During Prerender for Angular SSR  #3128). The reCAPTCHA providers need a browser.
• On main since fix(app-check)!: skip App Check during server rendering by default #3773, not yet in a published release, AngularFire no longer creates an App Check instance during server rendering. The guide does not say:
  • that inject(AppCheck) is then null on the server,
  • how to opt back in with APP_CHECK_ON_SERVER,
  • that Firebase requests made during server rendering carry no App Check token, so products that enforce App Check reject them.

Scope

Documentation only. #3256 asks for App Check documentation more broadly.