Create API Overview Docs

Right now, the Swagger UI and static docs generated by the code in both branches of this repo basically just document the resources.

Some additional text should be written that we can include at the top of the docs that discusses the following topics:

• Authentication - we should explain that we use basic auth and recommend best practices
• Basic query search - we should say that some resources support the "q" parameter (at least until RESTWS-561 is resolved)
• Representations - we should explain the concept of default, full and custom representations

The wiki page at https://wiki.openmrs.org/x/P4IaAQ has some of the content that should go in the overview docs.