Add explanation of dataset name interpolation (Pest PR #1146)

[JHWelch]

This PR adds documentation for the feature added in this PR

This change requires an image to be added. I'm not sure how the webp files referenced in the documentation are generated or stored. It does not seem to happen in repo.

This new addition requires a new image, the same size as the datasets-named.

You could generate it with the following test

it('validates the :dataset field', function ($field) {
    expect(true)->toBeTrue();
})->with([
    'first_name' => null,
    'email' => null,
]);

For example:

Thanks as always!

[nunomaduro]

can you add this, but without the webp stuff?