Build C4 system diagrams

[data-sync-user]

We have a couple of diagrams of our service, more or less:

• https://mozilla.github.io/application-services/book/design/components-strategy.html
• https://mozilla.github.io/uniffi-rs/latest/

Let’s cook up some C4 diagrams for consistency. It’s possible to do these in mermaid if they’re simple or we can use structurizr which is less fun but more customizable.

Here are some example diagrams Ben made before he left: https://mozilla.github.io/ecosystem-platform/reference/system-diagrams

Consider putting these in an ARCHITECTURE.md. From STOR-177:

{quote}Add an ARCHITECTURE.md (this post explains the idea behind it). This would explain the relationships between the crates in the repo and would generally just explain how everything is set up. We should use cargo depgraph to generate a diagram of the relationships between the crates{quote}

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