Adding unemployment rate at a census tract level

[Niklewa]

No description provided.

[rfl-urbaniak]

Just the same data guide structures as for the other PRs, more or less. Make sure the TOC makes sense, the new data groups are linked to in TOC in appropriate places, and that the TOC links lead where they are supposed to.

[rfl-urbaniak]

Comments in brackets in TOC are not uniform in style (time series/Time Series/Census Tract). I propose all of these start with lower case. Ditto for other notebooks.