docs: move the vpnkit internals diagram to the end
docs: move the vpnkit internals diagram to the end

Rather than dive into the middle of `vpnkit`, we start by looking
at the in-VM plumbing -- which is hopefully easy to understand --
and then zoom out to show how vpnkit can process this traffic.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit