Gather docs about internal design of workflow and workflow UI and publish them somewhere

[zach-klippenstein]

We've published some documentation on how to use workflow, but not how it works under the hood. See this slack thread.

[zach-klippenstein]

I think we have a lot of design-doc style material scattered across multiple issues and PRs over the last few years. It's there if you dig, but not discoverable at all.

[lucamtudor]

I've seen a couple of issues that discuss API designs, but they're hard to follow since I don't have all the context around them. It would be great to see some material in one place, or at least a list of issues to read in the correct order.