Add docs for attributes for each resource

Right now it's a bit hard to see what attributes are available on each resource.

For example, I'm moving https://github.com/pearkes/tugboat to use `droplet_kit`, but it's missing a few of the attributes on the image side, and I had to dig into the code to find out. Adding docs for each attribute, using the docs from the API DigitalOcean docs.

I did some of this in #115, but it would be nice to have full coverage! 😄

Start by reviewing the resource documentation and the DigitalOcean API documentation for each resource, using the work in #115 as a reference. Document the available attributes with full resource coverage, including the missing image attributes, and verify that the documented lists match the API.

Written by the indexing model from the issue text.