Clean up the "components" documentation

[shevron]

Now that we have written some howto guides / tutorials as part of #72 and #73, we should clean up the docs around Transfer adapters, Storage backends and Auth providers to make them more concise.

I'm thinking that each should be composed of a more "reference" style section, detailing each component type and each of the available options, and a "discussion" style section discussing design and abstractions.

Maybe, content that relates to creating custom components (if we have anything like that at all) should be moved to the development guide.