Update docs around Universal

Currently the [docs provided](https://github.com/angular/angularfire/blob/master/docs/universal/getting-started.md) for running universal with firebase have not been updated in over 2 years. Comparing the code provided there with the sample project in this repo, it's easy to spot some differences:

- major changes in [server.ts](https://github.com/angular/angularfire/blob/master/sample/server.ts), notably the lack of AppServerModuleNgFactory and a module mapper, the inclusion of a firestore "protos" loader
- the absence of a webpack config file
- the universal cloud function has changed
- package scripts are different

Given the long running issues and ongoing challenges of doing universal with firebase, it would be awesome to have current docs on how to set this up in a way that is proven to work. If you are open to PRs I could open one.

It may also be worth maintaining a separate repo for a sample project that demonstrates serving routes that use firestore data so we can be sure what works when server-side rendering pages that need firestore data. It would be a great reference but also a great testing ground for when new versions of angular or firebase are released.

Thank you!

### Version info

**Angular: 11**

**Firebase: 8.0.0**

**AngularFire: 6.1.4**

**Other (e.g. Ionic/Cordova, Node, browser, operating system):**

### How to reproduce these conditions

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**Steps to set up and reproduce**

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**Sample data and security rules**

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### Debug output

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### Expected behavior

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### Actual behavior

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Comienza con docs/universal/getting-started.md y compáralo con el sample/server.ts actual, los scripts del paquete, la configuración de webpack y la función cloud universal. Actualiza la documentación para que coincida con el proyecto de ejemplo, incluidos los cambios en server.ts y el Firestore protos loader; después, verifica que la configuración documentada de Firebase Universal funcione con la estructura actual del repositorio.

Escrito por el modelo de indexación a partir del texto del issue.