Add compatibility (docs + extra checks) for external data use

[emprzy]

This PR:

• Adds a documentation page (and reference to this page on the "getting started" page) to show users how they could use their own ground truth data as a parameter in get_fcast(). Note that using ones' own data is an alternative path to fetching NHSN data with our get_data().
• Adds a couple of extra safety checks to get_fcast() and to_respilens():
  • get_fcast(): explicitly requires that get_fcast() df parameter have target and location columns (in addition to target_end_date and observation)
  • get_fcast(): throws an error if there is more than one unique location or unique target in the df parameter
  • to_respilens(): throws an error if the model_out_tbl or oracle_output components of the parameter have more than one unique location

Separately, I also removed the codebase's dependency on loc_data. Because we want to aim to accommodate external data, we can't possibly have location metadata that covers every possible location. So, to_respilens() (which was the only thing that needed loc_data) simply just broadcasts the value in model_out_tbl$location[1] across every location-related metadata entry. MyRespiLens doesn't need abbreviation, location_name, location, and population to function correctly. It just needs a single location name.

[emprzy]

Linked to issue #42