Portable stories - Document common use cases and limitations

[yannbf]

We should improve the documentation of portable stories to provide different use cases, such as:

• Reusing the play function
• Using loaders
• Overwriting globals (e.g. rendering in english/spanish)
• Asserting spies from mocked args e.g.

  it('onclick handler is called', async () => {
    const onClickSpy = vi.fn();
    render(<Secondary onClick={onClickSpy} />);
    const buttonElement = screen.getByRole('button');
    buttonElement.click();
    expect(onClickSpy).toHaveBeenCalled();
  });

  // or
  it('onclick handler is called', async () => {
    render(<Secondary />);
    const buttonElement = screen.getByRole('button');
    buttonElement.click();
    expect(Secondary.args.onClick).toHaveBeenCalled(); 
  });

// the example above will work as long as the story has a vi.fn defined like so:
import { fn } from '@storybook/test';
CSF2Secondary.args = {
  onClick: fn(),
};

- Asserting events emitted (vue)

As well as limitations, such as:
- Story native formats (vue, svelte) are not supported