Document the architecture of the curriculum

[40thieves]

Which module(s) and week(s) does this change affect? Module(s): N/A Week(s): N/A

What is the work that needs to be done?

Add some documentation of how the curriculum works at a high level.

I think this is different from #129, which documents specific components available within the content. The goal here is how the whole thing hangs together.

Why is this work important to do?

It's helpful for newcomers to have some understanding, especially when fixing bugs. This is especially important in light of our partnership with Migracode as we'll hopefully have a lot of new folks looking to contribute!

Who might need to know about this change?

I'm guessing this is a job for @SallyMcGrath!