Add C++ implementation overview documentation

[rabail-aamir]

Description

This document provides an overview of how C++ support is implemented in Splashkit Online, including:

• Where C++ related components live in the repository
• Role of the CXXCompiler
• How Web Workers and the Clang backend work
• How C++ is compiled to WebAssembly (WASM)
• How browser runtime executes compiled programs
• How error handling works

It will help new contributors understand structured and flow of C++ backend and supports future improvement tasks.

Fixes # (issue)

Type of change

• [ ✔] Documentation (update or new)

How Has This Been Tested?

This change affects documentation only.
No code functionality was modified.

Testing Checklist

• [✔ ] Tested in latest Chrome
• [ ✔] Tested in latest Safari
• [ ✔] Tested in latest Firefox

Checklist

• [✔ ] My code follows the style guidelines of this project
• [ ✔] I have performed a self-review of my own code
• [✔ ] I have commented my code in hard-to-understand areas
• [✔ ] I have made corresponding changes to the documentation
• [ ✔] My changes generate no new warnings
• [ ✔] I have requested a review from ... on the Pull Request

[stelaukin]

I've left some suggestions about the phrasing of a few things and there are a couple of minor spelling and grammar issues.