Add clearer documentation

[liamhuber]

Implementing Macro as a walled garden and Node.pull has helped clarify some of the specifications and class interactions in my mind -- in particular, I think I can now say more clear things about the run cycle, the concept of "upstream", and the concept of "parent". This includes the ideas about intended use and Workflow being "parent-most" and living and flexible while Macro is more crystallized, efficient, and suitable for composition (since it does not have the requirement to be parent-most). Some of this needs to be translated into class and module docs, but I think a very simple version should be added to the README too.

Additionally, the example notebook needs to be broken down into something like a "quickstart" for people who just want to get running, and a deeper dive for people who want to quickly learn the ins and outs of the codebase.

[liamhuber]

Everything else on my v0.1.0 checklist is done, but I don't expect to do this one over the weekend so let's leave it until after the Monday meeting and then hopefully incorporate some feedback from the ADIS workshop into the learning material.