Update Windows guide to include Windows Firewall

As reported by lokodlare in our forum we should include information about Windows Firewall on our Windows instructions at https://community.torproject.org/relay/setup/guard/windows/, because our guide "sadly omits to even mention Windows’ integrated Firewall. Windows Firewall is enabled by default in all more recent versions of Windows (for good reason!) and you will need to create new rules for your tor relay(s) or bridge(s) to be reachable from the outside".