Add overview of architecture

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Most information from the presentation should be available

[cristeahub]

Have we got the actual presentation available somewhere?

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https://docs.google.com/presentation/d/1ATsyooth8RKDiwSHLiAGDzxKaM7eXXLlsWiG0mPtRWw/edit?ts=5625195c#slide=id.p