Use Signal Documentation app for source/signal info

[melange396]

Once its ready, use our signal documentation web application as an authority for source and signal information. It can replace the convoluted process we have in place now which maintains the info in a google spreadsheet, then transfers/syncs the spreadsheet data to CSV files in this repository.

CSV files are https://github.com/cmu-delphi/delphi-epidata/blob/dev/src/server/endpoints/covidcast_utils/db_signals.csv and https://github.com/cmu-delphi/delphi-epidata/blob/dev/src/server/endpoints/covidcast_utils/db_sources.csv

gdoc sync process and GH action are in https://github.com/cmu-delphi/delphi-epidata/blob/dev/tasks.py and https://github.com/cmu-delphi/delphi-epidata/blob/dev/.github/workflows/update_gdocs_data.yml

among other locations, files are used in dev/src/server/endpoints/covidcast_utils/model.py#L195-L236 and indirectly in dev/integrations/server/test_covidcast_endpoints.py#L313-L318