Add documentation on polymorphic relationships

[GromNaN]

MorphTo relations where added in https://github.com/mongodb/laravel-mongodb/pull/2608

Laravel documentation: https://laravel.com/docs/10.x/eloquent-relationships#polymorphic-relationships

Documentation to update: https://github.com/mongodb/laravel-mongodb/blob/4.1/docs/eloquent-models.md#relationships