Update runoff documentation in Calculations section

Current documentation is mostly copy-pasted from Water Watch. The workflow section in particular is lacking critical details, thinking in particular those described in !55 (closed). It would be awesome to see the logic described in a non-narrative format, e.g. as pseudocode or possibly even a snippet of the code itself? That may take some more thinking about how best to convey this complex information.

Also pulling in content from #66 (closed) which is closely related:

• In this section of the calculations page: https://doi-usgs.github.io/hyswap/meta/calculations.html#workflow-for-determing-weighting-of-streamgages-for-area-based-runoff-calculations
• fix md format for URL links
• if possible, link directly to the data sets used (this might be better in the geospatial-data-assembly repo long-term, but for now this seems like the simplest place to add details)