docs: network setup guide
docs: network setup guide

Add command for bringing up guest network. Network is by default down,
which prevents user from adding default gateway, returning error:
"Network is unreachable"

Signed-off-by: Peter Hrvola <dc6acce007006ef06aa38c72df61989d26de8d4a@hotmail.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker