[WIP] API Primer

An API Primer for the Apigility documentation.

- [ ] What is an API?
- [ ] Types of APIs (which will cover RPC and REST, and discuss the Richardson Maturity Model)
- [ ] Representation formats
  - [ ] Overview (cover XML, JSON, etc.)
  - [ ] JSON
  - [X] HAL
- [X] Error reporting (and specifically Problem Details for HTTP APIs, or application/problem+json)
- [X] HTTP negotiation (what happens if an HTTP request method is used that you do not support; how to report what HTTP request methods _are_ supported)
- [ ] Content Negotiation
- [ ] Validation (specifically, reporting validation errors)
- [ ] Versioning (how to version an API, and how to request different versions of an API)