Type checking nested navigator is missing StackParamList definition and usage

In the Nesting Navigators guide, the example given does not work with the stack structure provided in the page (which is not nested to begin with). I believe the author is implying a different nested stack (since the example is using StackParamList instead of RootStackParamList), but the definition for the StackParamList is not provided, confusing the reader into thinking it's the same as the RootStackParamList.

The definition of StackParamList and its usage should provided to give context to the nested examples.

開啟 versioned_docs/version-6.x/typescript.md，並搭配頁面上顯示的 stack 結構閱讀 Nesting Navigators 區段。新增缺少的 StackParamList 定義及其用法，讓巢狀範例與周圍的範例一致，且不必猜測就能理解。