Update Documentation to Clarify How Keys and Certificates Work

[psteininger]

In the past week or so I went through adding NervesHubLink and NervesKey to one of my projects and came away with some lessons I want to contribute to the documentation.

My feedback is that it is quite hard to wrap one's head around all the key/cert pairs and what role each one plays and which ones are needed based on whether NervesKey is used or not.

Since the docs are open source I will go ahead and work on improving the docs so that it's easier to comprehend the setup and the key/cert usage options.