Categorize input and output variables in docs

[MaxGhenis]

While policy and assumptions are grouped, inputs and outputs are one big list. Some organization would also help visually for scrolling, as reported in #2458.

This spreadsheet has all input and output variables (about 100 of each) with a column for category. Anyone can edit it. Once that's done then we can add it to the relevant files to make its way to the jb docs.

[MattHJensen]

Categorizing variables by tax form of appearance would be natural for many if not most of these.