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/end.md

@@ -19,6 +19,8 @@ Iterator end();
 ```cpp
 #include <XCEngine/Core/Event.h>
 
+using namespace XCEngine::Core;
+
 Event<int> event;
 event.Subscribe([](int v) { printf("Callback: %d\n", v); });