Update 'getting started'

Ok I've tried to make this clearer and shorter. Could probably still do with some work. It had a few bits of content about the details of the responses for each endpoint that I think should live on the specific pages unless it doesn't already, this should be a general overview, with only the most essential details, and a longer-form explanation of what the Content API is and does. Let me know what you think.

:sailboat: :fire_engine: