[SCOUT] Adding Declaring Model Engine to Scout docs

[JustSteveKing]

Every now and then (admittedly not very often) you might have different models registered on different search engines. This PR adds to the scout docs explaining how to override on a model basis the engine it is using - removing the need for hot swapping configuration at runtime.

<?php

 namespace App\Models;

use Illuminate\Database\Eloquent\Model;
use Laravel\Scout\EngineManager;
use Laravel\Scout\Searchable;

class User extends Model
{
    use Searchable;

    /**
     * Get the value used to index the model.
     *
     * @return \Laravel\Scout\Engines\Engine
     */
    public function searchableUsing()
    {
        return app(EngineManager::class)->engine('meilisearch');
    }
}