enhancement: align lab images/diagrams with actual examples

hi! :) first of all, i'm loving your labs.

i've been doing the labs on /courses/computer-networking-fundamentals/l3-to-l2-segment-mapping and find that some diagrams on what we are doing would help. there are some high level diagrams, but they don't match in IP subnet address, nor reflect the exercise itself. i painted it in some papers for myself, but maybe a more 'expert' diagram would help us also diagram better in the network context.