Add Mimir architecture overview diagram

[09jvilla]

Once the website team has produced an initial first draft of this diagram, we'll need technical review. Then we'll need to figure out were it goes in the docs.

Will be based on:
https://github.com/grafana/mimir/blob/main/docs/sources/images/architecture.png

See Monday board tracker:
https://grafana-labs.monday.com/boards/1266217351/pulses/2264442070

[09jvilla]

Now that we have an architecture overview diagram, I can replace the one that we have in 'about the architecture'. Diagram content is roughly the same, but the new diagram would match the format of our other diagrams.

[09jvilla]

assigning to myself