update endpoint documentations

[sgratzl]

the new flask server supports new version of the endpoints, e.g. /api.php?endpoint=covidcast -> will become /covidcast/.

Those new versions have new features:

• row limit up to 1M rows
• delphi epidata return format, will always return an epidata element even if there are no rows given
• better formats: ?format=csv, ?format=json which return plain versions and use HTTP status codes to communicate errors (e.g. 400 = bad request, in case of wrong parameter) -> I would consider ?format=json the better default
• filter returned fields (include and exclude fields)
• /version endpoint to show server information

In case of covidcast

• new ways of specifying parameters: see [flask]: add new parameter version: signal,time,geo #481 -> query multiple sources/signal, multiple geo_types/values, times ... at once
• new customized endpoints:
  • /meta ... alternative meta data format with more meta data about signals
  • /trend ... simple trend computation for a given date based on a given window
  • /trendseries ... trend computation for a series of dates at once
  • /correlation ... correlation between one and other signals
  • /backfill ... backfill information about all the issues of a signal