docs: Add Component, GameObject, TransformComponent and Scene API documentation

- Add Component class documentation with lifecycle methods
- Add GameObject class documentation with component system
- Add TransformComponent documentation with transform methods
- Add Scene class documentation with GameObject management
- Add SceneManager singleton documentation with scene loading
- Update components.md overview with all component classes
- Update main.md navigation with Scene module

docs/api/components/game-object/get-child.md

@@ -0,0 +1,17 @@
+# GameObject::GetChild
+
+获取指定索引的子对象。
+
+```cpp
+GameObject* GetChild(size_t index) const;
+```
+
+**参数：**
+- `index` - 子对象索引
+
+**返回：** `GameObject*` - 指定索引的子对象，如果索引无效则返回 nullptr
+
+## 相关文档
+
+- [GameObject 总览](game-object.md)
+- [GetChildren](get-children.md)