Update the cursor pagination docs to use the new Eloquent cursor implementation

[lindyhopchris]

We need to update the cursor pagination section so that it uses the new Eloquent implementation that was added via laravel-json-api/eloquent#37

We should:

• Copy and paste the existing cursor docs into the readme in the cursor pagination package.
• Add a section to that readme about how to migrate from one to the other. (Making it clear that this can be done in a non-breaking way as long as you haven't told clients they can just use a plain ID for the cursor, as that will be the only thing that doesn't work.)
• Update the existing cursor docs to instead talk about the new Eloquent implementation.
• Add an info box to the existing docs that make it clear that people looking for the old package should go to the readme in that repo.