docs: update memory and threading API docs

docs/api/memory/memorymanager/createlinearallocator.md

@@ -0,0 +1,32 @@
+# MemoryManager::CreateLinearAllocator
+
+```cpp
+std::unique_ptr<LinearAllocator> CreateLinearAllocator(size_t size);
+```
+
+创建并返回一个新的 LinearAllocator 实例，使用系统分配器作为底层。返回的 `unique_ptr` 管理分配器生命周期。
+
+**参数：**
+- `size` - 分配器缓冲区大小（字节）
+
+**返回：** LinearAllocator 的 unique_ptr
+
+**复杂度：** O(1)
+
+**示例：**
+
+```cpp
+#include <XCEngine/Memory/MemoryManager.h>
+
+auto linear = MemoryManager::Get().CreateLinearAllocator(1024 * 1024);
+void* ptr = linear->Allocate(256);
+void* marker = linear->GetMarker();
+linear->Allocate(128);
+linear->SetMarker(marker);
+linear->Clear();
+```
+
+## 相关文档
+
+- [MemoryManager 总览](memorymanager.md) - 返回类总览
+- [LinearAllocator](../linear-allocator/linear-allocator.md) - 线性分配器