Update twoliter design doc to reflect current design

[sam-berning]

There are a few differences between the design doc at docs/design/README.md and the actual implementation to enable kits. We should update the design doc to reflect the actual design.

Definition of done

The design doc should be changed with:

• More accurate definitions/example kits
• Updated description of the update/fetch process

[sam-berning]

I began work on this but I need to drop it for now to focus on kit release automation. My work-in-progress can be found at https://github.com/sam-berning/twoliter/tree/update-design-doc