Documentation issues

### Summary

The `launch.json` fails to work for two reasons:

1. It expects 'miDebuggerPath' to be supplied even if `gdb.exe` is on the path.
2. The `PATH` environment variable is set incorrectly and this ends up blatting the variable entirely causing the debugger fails to launch.

Also in the quickstart guide you might want to suggest new users use MYSYS2 instead of Mingw-w64 as C++17 is completely broken in Mingw-w64 (https://sourceforge.net/p/mingw-w64/bugs/737/) and rarely updated, whereas MYSYS2 is actively maintained.

### Platform and Versions

- **Operating System**: Windows 10
- **VSCode Version**: 1.45.1
- **CMake Tools Extension Version**: 1.4
- **Compiler/Toolchain**: GCC

### Fixes

Modify `launch.json`:

```
"environment": [
{
"name": "PATH",
"value": "${env:PATH};${command:cmake.launchTargetDirectory}"
}
],
"miDebuggerPath": "Path to gdb"
```

Review the launch.json configuration and the quickstart guide mentioned in the issue, starting with the existing debugger and PATH examples. Confirm the Windows GCC setup works when gdb is on PATH, then update the configuration guidance and MSYS2 recommendation so the documented launch flow works.

Written by the indexing model from the issue text.