Move registry/architecture.md back to docker/distribution

[stevvooe]

This is mostly just a few architectural notes that aren't even published. Let's move this back.

https://github.com/docker/docker.github.io/blob/master/registry/architecture.md for reference.

[mdlinville]

Now that distribution/distribution#2282 and distribution/distribution#2284 are merged, I can do the docs part of this.

[docker-robott]

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