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/size.md

@@ -0,0 +1,44 @@
+# String::Length / Capacity / Empty
+
+```cpp
+SizeType Length() const;
+SizeType Capacity() const;
+bool Empty() const;
+```
+
+获取字符串的长度、容量和判空状态。
+
+**参数：** 无
+
+**返回：**
+- `Length()` - 返回字符串的字符数（不包括终止 null 字符）
+- `Capacity()` - 返回已分配的存储容量
+- `Empty()` - 如果字符串为空则返回 `true`
+
+**复杂度：** 均为 O(1)
+
+**示例：**
+```cpp
+#include "XCEngine/Containers/String.h"
+#include <iostream>
+
+int main() {
+    XCEngine::Containers::String s1;
+    std::cout << "Empty: " << s1.Empty() << std::endl;  // 输出: Empty: 1
+
+    XCEngine::Containers::String s2("Hello");
+    std::cout << "Length: " << s2.Length() << std::endl;      // 输出: Length: 5
+    std::cout << "Capacity: " << s2.Capacity() << std::endl;  // 输出: Capacity: 6 或更大
+
+    s2.Reserve(100);
+    std::cout << "After Reserve(100), Capacity: " << s2.Capacity() << std::endl;  // 输出: 100
+
+    return 0;
+}
+```
+
+## 相关文档
+
+- [String 总览](string.md) - 返回类总览
+- [CStr](cstr.md) - 获取 C 字符串
+- [Reserve / Resize](reserve-resize.md) - 内存管理