docs: Fix core module documentation discrepancies

Fixed the following issues in XCEngine Core module documentation:
- Added 'using namespace XCEngine::Core;' to all code examples that use
  Core types (Event, FileWriter, etc.) without full namespace qualification
- Added missing '#include <XCEngine/Containers/String.h>' to FileWriter
  examples that use Containers::String
- Added '#include <string>' to Flush.md example using std::to_string

Affected files:
- core/core.md: Added using directive and Containers include
- event/*.md: Added using namespace to all 8 event doc files
- filewriter/*.md: Added using namespace and proper includes to all 6 files

docs/api/core/event/Subscribe.md

@@ -22,6 +22,8 @@ uint64_t Subscribe(Callback callback);
 ```cpp
 #include <XCEngine/Core/Event.h>
 
+using namespace XCEngine::Core;
+
 // 定义事件
 Event<int, float> damageEvent;