Addon-backgrounds: Migrate to story globals

[shilman]

Describe the problem

Story globals simplifies how to configure globals at the story level. We should update the Backgrounds addon.

• Figure out a final API
  • globals for storing/accessing state
• Update documentation

Before

export const Story = {
  parameters: {
    backgrounds: {
      default: 'red',
      grid: { ... },
    },
  },
};

After (TBD)

export const Story = {
  globals: {
    'backgrounds.value': 'red',
    'backgrounds.grid': true,
  },
};

Additional context

No response