Developer onboarding quest

[lindanlee]

Task: build out a kind of "developer onboarding quest" - a bunch of tasks one can do, following the documentation, to help familiarize yourself with zcash, and it occurred to me that it might be useful to have a similar sort of thing in the readthedocs site - a quest/set of challenges that one could complete to really get going with zcash.

Context: Kris already has some of this content written, documentation can assist it over the finish line and take care of uploading it.

Done criteria: developer onboarding quest/guide is live on read the docs.