repo architecture

[hturner]

This was originally part of #42.

Comments from @mikerspencer

Repo structure

• collect together similar files (R code, data) into directories
• add build/workflow diagram to readme (e.g. with mermaidjs)

Since the two dashboards are based on different data sources, there won't be much overlap in the R code/data sets, but it could be helpful to reorganize the repo to work towards better integration of the two dashboards.

Refactoring the code, as suggested in #8 may enable the dashboards to use the same R functions, where that makes sense.