Update style on core workflow image and light/dark theme

[joshuaogle]

trafficstars

Updated some docs images to new style and to add light/dark theming.

• boundary-network.png image was originally made to replace the production.png but it looks like we missed one. It isn't in the new style, but looks a lot better.
• access-model image was the same as how-boundary-works, so I replaced that too