docs: Add platform setup instructions for macOS, Windows, and Linux
- Updated step 3 in setup_run.md to include platform-specific instructions for enabling Windows, macOS, or Linux based on the user’s desktop environment.
- Added `flutter create --platforms=<platform>` command example for clarity.
- Added a reference to `platform_specific_instructions.md` to guide users on configuring platform-specific requirements after setup.
This update clarifies setup steps and reduces confusion for new contributors
by centralizing necessary setup instructions in one place.