Document addon configuration fields for Replicator/Bard sets

[eminos]

Companion tracking issue for the proposed API in statamic/cms#15422. The PR is a draft and has not been merged.

The change adds Statamic\Fieldtypes\Sets::appendSetConfigField($handle, $config) and appendSetConfigFields($fields) to register native configuration fields in the Edit Set stack for Replicator and Bard.

Documentation should distinguish reusable set configuration in a blueprint/fieldset from fields stored on individual content blocks. For example, an addon can register an editorial note or a native group of guidance fields. The values are saved alongside that set's native configuration.

Suggested coverage once the API is accepted:

• Register fields during a service provider's boot phase, using keyed native field definitions.
• Single-field and multiple-field examples, including a group for namespaced nested configuration.
• Native preprocessing, metadata, validation, and processing apply. Validation is enforced when applying/saving the containing field.
• Edit Set's Confirm button updates the containing field editor; save the field and blueprint/fieldset to persist changes.
• Native set keys are reserved. Use addon-specific handles.
• Existing custom metadata remains intact when its addon is not registered.

The extension is generic; no AI-specific settings are added to core.