Documentation update for smoothed safegraph signals

[sgsmob]

This PR updates the signal documentation to include smoothed version of the safegraph signals. See cmu-delphi/covidcast-indicators#332.

[capnrefsmmat]

It may be worth adding, after the signal list and the paragraph on estimation, a short paragraph saying why 7-day averaging is useful. Something like:

Because these signals measure mobility each day, there are strong day-of-week effects: weekends have substantially different values than weekdays. Users interested in long-term trends, rather than mobility on one specific day, may prefer the 7d_avg signals, since averaging over the preceding 7 days removes these day-of-week effects.

[sgsmob]

It may be worth adding, after the signal list and the paragraph on estimation, a short paragraph saying why 7-day averaging is useful. Something like:

Because these signals measure mobility each day, there are strong day-of-week effects: weekends have substantially different values than weekdays. Users interested in long-term trends, rather than mobility on one specific day, may prefer the 7d_avg signals, since averaging over the preceding 7 days removes these day-of-week effects.

Thanks, done.