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/resources/resourcehandle/isvalid.md

@@ -0,0 +1,29 @@
+# ResourceHandle::IsValid
+
+```cpp
+bool IsValid() const
+```
+
+检查句柄是否持有有效资源。判断条件为：内部指针非空且资源的 `IsValid()` 返回 true。
+
+**参数：** 无
+
+**返回：** 如果持有有效资源则返回 true，否则返回 false
+
+**复杂度：** O(1)
+
+**示例：**
+
+```cpp
+ResourceHandle<Texture> tex = ResourceManager::Get().Load<Texture>("tex.png");
+if (tex.IsValid()) {
+    // 安全访问资源
+    tex->GenerateMipmaps();
+} else {
+    printf("Texture load failed!\n");
+}
+```
+
+## 相关文档
+
+- [ResourceHandle 总览](resourcehandle.md) - 返回类总览