Redo of #10: Initial documentation and data fetching guide (with examples)

[krivard]

#10 got clobbered by more-recent work so I resolved conflicts but got turned around part way through and now there's a local branch that should probably be reviewed by someone with more recent experience in epidat*. Probably low-pri?

Woochan's original text below:

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Summary:

This PR aims to add a basic starter documentation in Sphinx to help understand what this package does and how it works. The main focus is on using the Epidata object that covers all the API endpoints. More work should be done to understand the COVIDcast segment of the code (which seems to be an extra add-on).

• Added data source tables containing appropriate descriptions and hyperlinks leading to the main API documentation
• Briefly tested the "EpiRange" parameter and added a short guide to the documentation
• Added examples of API call and data parsing (CSV, JSON, Classic, etc.) for:
  • covidcast
  • wiki
  • fluview
• Drafted data fetching guide using the Epidata object
• Added descriptions and parameters for Epidata signal functions
  • Currently implemented for covidcast. If this is the correct direction, need to be done for all functions
• "More on COVIDcast" section is written by a previous member. Will need to confirm if this section is needed.