Add documentation for API users

[bfreeds]

Is your feature request related to a problem? Please describe.
Documentation of how to use Layers API. Current documentation is more geared at developers of layers-api, not users of the api.

Describe the solution you'd like

• "Getting Started" and "Usage" sections that has
  • example request and responses (which show the Schemas, rather than pointing to the code)
    • 
  • documents all available layer groups currently in the api
• GSA Example of API Documentation

Describe alternatives you've considered

• Postman Collection & generated Documentation
• Swagger

Additional context
Our team is using the free tier of Postman currently, so using it for this purpose would bump us up to a paid plan.