Provide documentation for API Integration

[ohader]

• document scope, functionality and usage of the the API project
• focussed on
  • integrators (preliminary)
  • editors (if it makes sense, describe the limits, e.g. "editor needs to have templating knowlege)
• use an example (e.g. league results of a soccer team) and demonstrate step-by-step guide
  • how to create endpoint, base and field-mapping
  • how to write a (simple) Fluid template example
  • → actual result in the frontend
  • (use screenshots where applicable)

Side-node: Avoid providing real API tokens or credentials in documentation of screenshot (either redact them or use bogus/imaginary credentials)