Add updated auth diagrams

Our auth diagrams are no longer up-to-date with our current auth flow. We need updated versions of sample auth flows in the README.

Below is a modern example of the auth flow. The new charts should be generic in the way that services are generically named to show a user how the auth flow with auth.abandontech.cloud will fit into their applications. We may wish to include two versions--one version for a server-side rendered application, and an additional version for an application with separate front/backends. The differences between these two are subtle, but I believe it is worth illustrating both.

Diagrams should be clearly labeled and color-coded similarly to the existing ones in the readme