Improve readme / KB

I encountered some difficulties when I was configuring AdGuard CLI via VPN CLI in SOCKS5 proxy mode:

- how to configure `outbound_proxy` - this feature is mentioned in the README and KB, but it is not clear how to configure it.
**Answer:**
1. Turn on outbound proxy:
`adguard-cli config set outbound_proxy true`
2. Configure outbound proxy **mode**, **host**, **port** (AdGuard VPN CLI or another proxy)
IP and port - for reference.
* mode - HTTP or Socks 5. The same type as the outbound proxy
`adguard-cli config set outbound_proxy.mode socks5`
* outbound proxy address
`adguard-cli config set outbound_proxy.host 192.168.88.100`
`adguard-cli config set outbound_proxy.port 1080`

- it is not described how to download the AdGuard CA certificate on a remote device, connected to AdGuard CLI.

**Answer:**
1. Connect the client device to the proxy
2. Open `http://local.adguard.org/cert` in a browser and download the file
3. Install it in the root certificate store (link to instructions for Windows, Android, Linux) or to the storage of a browser (Firefox)

CC: @el-termikael