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

首先定位 AngularFire 现有的产品文档文件夹，并阅读 issue #2858 和 discussion #3194，了解报告中关于 AppCheck 集成的困惑。记录如何通过 AngularFire 集成和使用 AppCheck，包括它是否适用于 compat API；当两个问题都得到清晰回答时，工作即告完成。

由索引模型根据 Issue 内容生成。