docs: add Windows, Linux, and self-hosted runner examples to README

[eduardofraga]

Replace the minimal example usage section with three complete, copy-paste-ready examples:

1. Windows via WinRM — INI inventory, win_ping playbook, password via secret
2. Linux via SSH — YAML inventory, SSH key injection, playbook + ad-hoc command
3. Self-hosted runner — internal network targets, health check playbook, runner-only-needs-Docker