Do a better job of explaining what resources and representations are

[mstade]

See this comment (and thread) for some backstory.

We need to better explain that resources are concepts, something abstract to talk about a thing in general terms. We also need to better explain that representations are concrete "renderings" of that resource; the meaning of which is implementation dependent.

For example, a JSON representation might be a raw data dump, but the underlying data source may be in a different format entirely, so it's still a "rendering". Even if it was a simple pass-through, it should still be considered a "rendering" of the resource, which should never really talk about formats when discussing the "thing" – only when you want to represent it.

This is a confusing topic for many, so should be fleshed out a lot. Preferably with a bunch of demos as well.