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/operator-subscript.md

@@ -0,0 +1,51 @@
+# String::operator[]
+
+```cpp
+char& operator[](SizeType index);
+const char& operator[](SizeType index) const;
+```
+
+通过索引访问字符串中的字符。
+
+**参数：**
+- `index` - 要访问的字符位置（从 0 开始）
+
+**返回：** 位置 `index` 处字符的引用（可写或只读）
+
+**复杂度：** O(1)
+
+**注意：** 不进行边界检查。调用者需确保 `index < Length()`。
+
+**示例：**
+```cpp
+#include "XCEngine/Containers/String.h"
+#include <iostream>
+
+int main() {
+    XCEngine::Containers::String s("Hello");
+
+    // 只读访问
+    for (XCEngine::Containers::String::SizeType i = 0; i < s.Length(); ++i) {
+        std::cout << s[i];
+    }
+    std::cout << std::endl;  // 输出: Hello
+
+    // 可写访问
+    s[0] = 'J';
+    s[1] = 'a';
+    s[4] = '!';
+    std::cout << s.CStr() << std::endl;  // 输出: Jallo!
+
+    // const 版本
+    const XCEngine::Containers::String& cs = s;
+    char first = cs[0];
+    std::cout << "First char: " << first << std::endl;  // 输出: First char: J
+
+    return 0;
+}
+```
+
+## 相关文档
+
+- [String 总览](string.md) - 返回类总览
+- [Length](size.md) - 获取长度