docs: Add architecture diagram

Signed-off-by: Chris Crone <christopher.crone@docker.com>

docs/architecture.md

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+# Architecture
+
+This CLI has the following high level design goals:
+1. Provide a way for the Docker experience to be mapped to different container
+   runtimes
+1. Provide a way to automatically generate high quality SDKs in popular
+   languages
+1. Ensure that existing Docker CLI commands continue to work as before
+
+These constraints resulted in the following architecture:
+
+![CLI architecture](./images/cli-architecture.png)
+
+What follows is a list of useful links to help navigate the code:
+* The CLI UX code is in [`cli/`](../cli)
+* The backend interface is defined in [`backend/`](../backend)
+  * An example backend can be found in [`example/`](../example)
+* The API is defined by protobufs that can be found in [`protos/`](../protos)
+* The API server is in [`server/`](../server)
+* The context management and interface can be found in [`context/`](../context)
+* The Node SDK is autogenerated and can be found in
+  [`docker/node-sdk`](https://github.com/docker/node-sdk)

docs/images/cli-architecture-src.drawio

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