doc: Document how to change the Cockpit TCP port

I use this:

```
# make cockpit listen on a different port
cockpitPort="9091"
cockpitSocketFile="/etc/systemd/system/cockpit.socket"

semanage port -a -t websm_port_t -p tcp "$cockpitPort"
rm -f "$cockpitSocketFile"
cp /lib/systemd/system/cockpit.socket "$cockpitSocketFile"
sed -i "s/9090/$cockpitPort/" "$cockpitSocketFile"
systemctl daemon-reload
systemctl restart cockpit.socket
for i in "" "--permanent"; do \
  firewall-cmd \
    --add-port="$cockpitPort/tcp" \
    $i; \
done
```