Describe system and user-mode networking

... and how to change this.

Especially as this is now the default for all platforms, including Linux.

Ref: https://github.com/crc-org/docs/pull/25

---

### What is the difference

 - binds on `localhost` host, therefore allows use with most VPN route-all solutions


### How to change
```
$ crc config set network-mode system
```