doc/dev: add Code Insights developer docs
Created by: slimsag
This adds a few things:
- Describes the current state of the MVP backend at a high-level.
- Adds an architecture diagram.
- Adds a "Life of an insight" section which walks through how the whole system works end-to-end.
- Adds a debugging section which covers all development debugging tips I have.
- Describes creating DB migrations briefly.
Fixes #18965 (closed)
One thing this does not yet cover is various design decisions and tradeoffs we weighed. I am going to add that in a subsequent PR, as that is probably one of the more useful things for a new hire to have.
Signed-off-by: Stephen Gutekanst [email protected]