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

We need to update the cursor pagination section so that it uses the new Eloquent implementation that was added via https://github.com/laravel-json-api/eloquent/pull/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.

laravel-json-api / laravel-json-api.github.io