Add docs on customizing Blueprint and Fieldset repositories.

[joshuablum]

Like doing something like this in our AppServiceProvider for Fieldsets:

$this->app->singleton(\Statamic\Fields\FieldsetRepository::class, function () {
  return (new \Statamic\Fields\FieldsetRepository)
    // define the path here
    ->setDirectory(resource_path('fieldsets'));
});

or this for Blueprints:

$this->app->bind(\Statamic\Fields\BlueprintRepository::class, function () {
  return (new \Statamic\Fields\BlueprintRepository)
    ->setDirectory(resource_path('blueprints'))
    ->setFallback('default', function () {
      return \Statamic\Facades\Blueprint::makeFromFields([
        'content' => ['type' => 'markdown', 'localizable' => true],
      ]);
    });
});

Took statamic/cms#8344 for reference.