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

首先找出 AngularFire 現有的產品文件資料夾，並閱讀 issue #2858 和 discussion #3194，了解所回報的 AppCheck 整合困惑。記錄如何透過 AngularFire 整合及使用 AppCheck，包括它是否能與 compat API 搭配運作；當兩個問題都獲得清楚回答時，工作即告完成。

由索引模型根據 Issue 內容生成。