Add dev documentation for workflow operations

[cwilby]

I wrote this while learning about the workflow engine so I could refactor a media converter I was building to use that rather than timed background jobs and a bunch of config JSON nonsense.

I saw that documentation was needed, so I watched/read through blizzz's talk and read the code for the workflow engine, and wrote this up to help build my own understanding.

[cwilby]

I'll likely come back and edit this once I finish with the app.