Add C++ implementation overview documentation

Documentation/cpp-overview.md

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+# C++ Implementation Overview in SplashKit Online
+
+## Introduction
+
+This document gives an overview of how Splashkit Online compiles and executes code. It describes how user-written C++ code flows from the browser editor to WebAssembly compilation and execution.
+
+The main purpose is to help new contributors understand how the system integrates C++ capabilities without requiring in-depth compiler knowledge.
+
+## Where C++ Lives in Repository
+
+- **compilers/cxx:**  
+Includes the primary logic for applying Clang to compile C++ code to WebAssembly.
+- **runtimes/cxx:**
+Manages the browser's runtime execution of C++ programmes that have been compiled.
+- **SplashKitWasm:**
+Contains the WebAssembly build of the SplashKit library used by compiled programs.
+- **javascript/compilers/cxxCompiler.js:**
+Manages the front-end compilation process.
+
+## Role of CXXCompiler
+
+The **CXXCompiler** class manages the C++ compilation process. Its responsibilities include:
+
+- Writing source files to the virtual filesystem
+- Compiling each C++ file into object files
+- Linking object files into final WebAssembly output
+- Providing syntax checking and error reporting
+- Warning users about unsupported API usage
+
+It communicates with a Web Worker to ensure the main interface doesn't freeze during compilation.
+
+
+## Web Workers and Clang Backend
+
+In a Web Worker, the actual compilation process is carried out using:
+
+- **clang++.wasm**: Compiles C++ source into object files
+- **wasm-ld.wasm**: Links object files into a final 
+
+This occurs asynchronously to maintain UI responsiveness.
+
+## Relationship with WebAssembly (WASM)
+
+SplashKit Online can securely run C++ code within the browser thanks to WebAssembly.
+The browser's runtime interprets WASM, which is created by the compiler from C++.
+This method offers:
+
+- Sandboxing for security
+- Compatibility between platforms
+- Real-time execution in web environments
+
+## Execution Environment
+
+Runtime environment handles:
+
+- Program start / stop / pause
+- File system interactions
+- Canvas rendering
+- Audio output
+- Keyboard and user input
+
+The ExecutionEnvironmentInternalCXX class is in charge of this, coordinating communication between the browser user interface and the compiled programme.
+
+## Error Handling and Output
+
+During error handling, compiler messages are processed and formatted before being displayed to the user. 
+This is done to ensure key lines are emphasised and clear terminal feedback is provided through error analysis.