allow linked-data data source documentation

[ksonda]

The data sources listed at https://labs.waterdata.usgs.gov/api/nldi/linked-data?f=json have short, esoteric names that are insufficient to understand if a particular data source meets a use case, or what additional filtering might be required or third-party web services might be available after an NLDI response to meet a use case.

I think it would be good to allow data contributors to specify a URL pointing to a web resource that provides more detailed and useful metadata about the data source.

Like

{
"source": "gfv11_pois",
"sourceName": "USGS Geospatial Fabric V1.1 Points of Interest",
"features": "https://labs.waterdata.usgs.gov/api/nldi/linked-data/gfv11_pois",
"metadata": "https://doi.org/10.5066/P971JAGF"
},