Improve the English Quick Start documentation

[primaryNK]

Goal

Improve the English Quick Start documentation so that a new FlyArena user can
download, set up, and run the project without needing to inspect the source code.

Tasks

• Review the current README and getting-started documentation.
• Add or improve English instructions for the first launch.
• Explain the difference between SETUP_DATA_AND_RUN.bat and
  START_FLYARENA.bat.
• Explain why BANC data is downloaded separately.
• Keep all commands and filenames consistent with the current release.
• Do not change the technical meaning of the existing documentation.

How to verify

Follow the resulting instructions from a clean extracted Release directory and
confirm that all referenced paths, buttons, and commands exist.

Contributing

Pull Requests are welcome.

Please read CONTRIBUTING.md and the Contributor License Agreement before
starting.