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


Review the existing auth diagrams in the README and the example image provided in the issue. Create updated diagrams that illustrate generic auth flows for server-side rendered apps and separate frontend/backend apps, matching the style and color-coding of the current ones. Ensure the diagrams are clearly labeled and fit the project's documentation standards.

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