NHSN documentation needs cleaning

[carlynvandyke]

• Title is listed as "National Syndromic Surveillance Program Emergency Department Visits"
• In the Data Sources and Signals menu it's listed as "NHSN ED Visits"
• We should say more prominently that this data source is the continuation of the old HHS source

[melange396]

• NHSN documentation fixes for page title and heading #1588 took care of part of the requirements in this issue, but we still need some text (describing the relationship with the old "HHS" signal) for the "Overview" section in docs/api/covidcast-signals/nhsn.md

[melange396]

• very related issue: Mention NHSN in covid_hosp* and covidcast hhs docs #1352