fix: improve doc link navigation and tree display

- Fix link resolution with proper relative/absolute path handling
- Improve link styling with underline decoration
- Hide leaf nodes from tree, only show directories
- Fix log file path for packaged app

docs/api/containers/string/to-lower-upper.md

@@ -0,0 +1,36 @@
+# String::ToLower / ToUpper
+
+```cpp
+String ToLower() const;
+String ToUpper() const;
+```
+
+将字符串转换为小写/大写形式，返回一个新的 String 对象，原字符串不变。
+
+**参数：** 无
+
+**返回：** 转换后的新 String 对象
+
+**复杂度：** O(n)，其中 n 为字符串长度
+
+**示例：**
+```cpp
+#include "XCEngine/Containers/String.h"
+#include <iostream>
+
+int main() {
+    XCEngine::Containers::String s("Hello World 123");
+
+    XCEngine::Containers::String lower = s.ToLower();
+    std::cout << lower.CStr() << std::endl;  // 输出: hello world 123
+
+    XCEngine::Containers::String upper = s.ToUpper();
+    std::cout << upper.CStr() << std::endl;  // 输出: HELLO WORLD 123
+
+    return 0;
+}
+```
+
+## 相关文档
+
+- [String 总览](string.md) - 返回类总览