Share public version of MagicBox diagram on ReadTheDocs

[jwflory]

@mikefab has a full diagram of how the pieces of MagicBox fit together and what the components of the project are; we need to make a public version of that document and add it to our documentation

What to do

1. Create a public version of the graph with sensitive information or resources removed
2. Submit a pull request to unicef/magicbox with the picture (maybe to the index page underneath the table of contents section for now?)
3. Show diagram in the MagicBox README

Why do it

MagicBox is a complex project with many pieces. We want contributors to spend time working on valuable areas of MagicBox that help the project. By investing time and effort in the project, we want to reward that time and effort with accepted contributions. To accept meaningful contributions, we need to make it more clear how MagicBox fits together and what the "core" components are.