Mention NHSN in covid_hosp* and covidcast hhs docs

[brookslogan]

NHSN rather than HHS Protect now runs collection/aggregation/administration of this data set. To enhance searchability we should at least mention it in the associated documentation pages (and maybe the documentation endpoint docs?). And maybe think a bit more about the associated sidebar label for covidcast hhs ("Department of Health & Human Services"). Endpoint name is also now suboptimal but it's probably trickier and messier to do anything about that (alias? deprecate & change?).