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

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