docs: update math API docs

docs/api/math/viewport/getrect.md

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+# Viewport::GetRect
+
+```cpp
+Rect GetRect() const;
+```
+
+将视口转换为 Rect。
+
+**返回：** 对应的 Rect（x, y, width, height），不包含深度范围
+
+**线程安全：** ✅
+
+**复杂度：** O(1)
+
+**示例：**
+
+```cpp
+#include "XCEngine/Math/Rect.h"
+#include <iostream>
+
+using namespace XCEngine::Math;
+
+int main() {
+    Viewport viewport(0.0f, 0.0f, 1920.0f, 1080.0f, 0.0f, 1.0f);
+    Rect rect = viewport.GetRect();
+    std::cout << "Rect: (" << rect.x << ", " << rect.y << ", " << rect.width << ", " << rect.height << ")\n";
+    return 0;
+}
+```
+
+## 相关文档
+
+- [`Viewport::GetAspectRatio`](getaspectratio.md) - 获取宽高比
+- [Viewport 总览](viewport.md)