feat: add macOS and Linux setup guides

[DiogoRibeiro7]

Proposal

The course only provides Windows setup instructions (`course/00_setup/00_mysql_install_windows.md`). The README states "Environment: Windows" but the CI runs on Linux and scripts exist for both bash and PowerShell.

Add setup guides for:

• macOS: `brew install mysql`, MySQL Workbench download, connection setup
• Linux (Ubuntu/Debian): `apt install mysql-server`, alternative GUI tools

Why

University students use a mix of operating systems. Limiting the course to Windows excludes macOS/Linux users or forces them to figure out installation on their own.

Scope / files impacted

• `course/00_setup/00_mysql_install_macos.md` (new)
• `course/00_setup/00_mysql_install_linux.md` (new)
• `README.md` (update Environment section)