Update device agent network requirements to also cover self hosted

### Description

The device agent page only mentions FFC network hostname requirements, not self hosted

https://flowfuse.com/docs/device-agent/install/overview/#networking-requirements

e.g.

> But for a self hosted FlowFuse deployment you will require the following (assuming example.com as the domain)
>
>
> - forge.example.com (the host name that is used to access the FlowFuse web app, self hosted deployments tend to use `forge` not `app`)
>
> - mqtt.example.com that points to the MQTT broker included in the FlowFuse deployment
>
> - npm.example.com this is only required if you are using the Team Library custom nodes feature