Fill out `inputs/*/README.md` documentation

[kennedy-mindermann]

Summary

docs/sources.csv previously held a single-line description for each input file. This has been replaced with inputs/*/README.md per folder, and the prior sources.csv entries for inputs have been migrated in as a starting point. Most of the READMEs still need more complete information.

What a "complete" README covers

At a minimum:

• A description of each file in the folder
• Where the data comes from
• Any switches that affect it

How to contribute

Pick a folder, comment on the issue or link a PR so others know it's in progress, and check it off once merged.

A good example of what the input READMEs should look like is inputs/transmission/README.md.

READMEs that need to be completed:

• capacity_exogenous/README.md
• climate/README.md
• ctus/README.md
• degradation/README.md
• `demand_response
• dgen_model_inputs/README.md
• disaggregation/README.md
• emission_constraints/README.md
• financials/README.md
• fuelprices/README.md
• geothermal/README.md
• growth_constraints/README.md
• hydro/README.md
• load/README.md
• national_generation/README.md
• plant_characteristics/README.md
• reserves/README.md
• sets/README.md
• shapefiles/README.md
• state_policies/README.md
• storage/README.md
• supply_curve/README.md
• techs/README.md
• temporal/README.md
• upgrades/README.md
• userinput/README.md
• valuestreams/README.md
• waterclimate/README.md