Add explanation of documentation information architecture in general

[jdbaldry]

From @chri2547

On a related note, part of what's missing in the toolkit is an explanation of the docs IA in general and how to build out the directory structure in a repo. We address this in the docs 101 training that Brenda and I put together, but that content hasn't made its way to the toolkit. A few slides starting here: docs.google.com/presentation/d/1_d_GF1W_8wa21g2HpIe-K4lY74z42g4Vq--A9i9nkN4/edit#slide=id.g156475cb98f_1_12