feat(vpn-port-forwarding): make documentation up-to-date

[pzmarzly]

I was setting up VPN port forwarding for my home lab recently, and observed many issues with the docs. I ultimately succeeded, but the information I needed was scattered between many files, issues in different repositories, and ultimately I had to read the source code of the implementation to understand what's going wrong.

In this PR, I habe

• updated the list of supported port forwarding providers
• fixed qBittorrent command to disable incompatible settings
• fixed qBittorrent command to work correctly when provider assigns you multiple ports
• added a link to upcoming Transmission command (the link there will need to be changed once the script is merged)
• documented what VPN_PORT_FORWARDING_LISTENING_PORT does - addressing Bug: VPN_PORT_FORWARDING_LISTENING_PORT not working gluetun#2520
• documented limits of PrivateVPN (port forwarding is only available for OpenVPN, not Wireguard) - addressing feat(privatevpn): support natively port forwarding gluetun#2285
• documented gluetun's Wireguard setup quirk (you have to use IP address instead of domain name) - addressing Feature request: Support for DNS domains in WIREGUARD_ENDPOINT_IP and wg0.conf gluetun#2680