Re-introduce updated architecture overview diagram
Created by: tsenart
This commit re-introduces an initial updated architecture overview diagram produced with Graphviz, with the intent of aiding the on-boarding of new team mates.
We should try to keep this up to date, so whenever a new service or link between services happens, try to remember to update this diagram. It's hard to enforce this so I think we need to rely on building this habit, so I updated the GitHub PR template to that effect.
@sourcegraph/code-intel @sourcegraph/core-services @sourcegraph/web @sourcegraph/distribution: Please suggest changes, updates, refinements. Thanks!