Add C++ implementation overview documentation

Documentation/cpp-overview.md

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+# C++ Implementation Overview in SplashKit Online
+
+## Introduction
+
+This document gives overview of how in Splashkit Online, code is  compiled and execuded. It describes how user-written C++ code flows from the browser editor to WebAssembly compilation and execution.
+
+Main purpse is to help new contributors understand that how the system integrates C++ capability 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
+
+CXXCompiler class is responsible for managing the C++ complition process. Its responsibilities include:
+
+- Writing source files to virtual filesystem
+- Compile each file of C++ into object files
+- Link object files into final WebAssembly output
+- Provide syntax checking and error reporting
+- Warn users about unsupported API usage
+
+It communicates with Web Worker to make sure that 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 WASM executable
+
+In order to maintain UI responsiveness, this occurs asynchronously.
+
+## 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
+- Execution in real time in a web setting
+
+## 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
+
+Prior to being displayed to the user, compiler messages undergo processing and formatting. In order to emphasise certain lines and offer insightful terminal feedback, errors are analysed.