Documenation for AppCheck

While most products have their own docs folder, there is currently no documentation about how to integrate and use AppCheck via AngularFire. Lately there has been a bit of confusion about that (see [this issue](https://github.com/angular/angularfire/issues/2858) or [this discussion](https://github.com/angular/angularfire/discussions/3194)).

**It would be great to add documentation about that.**

The documentation should also cover whether it can work together with the compat API.

### Version info

**AngularFire:** 7.4.1

Start by locating AngularFire’s existing product documentation folders and read issue #2858 and discussion #3194 for the reported AppCheck integration confusion. Document how to integrate and use AppCheck through AngularFire, including whether it works with the compat API; the work is done when both questions are answered clearly.

Written by the indexing model from the issue text.