Updated Intro to API documentation topic

I updated the Introduction to API documentation topic in my API doc course in a couple of significant ways. First, I expanded on the many different types of APIs out there (beyond REST). The list now native library APIs, SOAP APIs, RPC-based APIs, gRPC APIs, REST APIs, Voice Assistant APIs, and IoT APIs. I also incorporated research from Smartbear's impressive report called The State of API 2019, which is well worth a read on its own.