"Date of last change" dates for some COVIDcast-signals pages should actively link to changelog

[tinatownes]

For the following sources, the "Date of last change" display accurate dates, but these dates are not actively linked to the changelog at https://cmu-delphi.github.io/delphi-epidata/api/covidcast_changelog.html:

...delphi-epidata/api/covidcast-signals/doctor-visits.html
...delphi-epidata/api/covidcast-signals/google-symptoms.html
...delphi-epidata/api/covidcast-signals/hospital-admissions.html
...delphi-epidata/api/covidcast-signals/quidel.html

An example of an actively linked "Date of last change" is currently available in the bulleted main summary list for "COVID-19 Trends and Impact Survey" at .../delphi-epidata/api/covidcast-signals/fb-survey:

[dshemetov]

To add: I've been confused about whether this line refers to documentation changes or data changes before. It's definitely doc changes, but to save others confusion, would be nice to clarify.

[JavierMtzRdz]

In PR #1702, the changelog was moved to its corresponding pages and linked to the "Date of last data revision" as discussed in #1654.