docs(addon-backgrounds): add merging example
docs(addon-backgrounds): add merging example

- also change preview.js example to use CSF standard

storybooks/react-storybook,kadirahq/react-storybook,storybooks/storybook,storybooks/storybook,storybooks/react-storybook,kadirahq/react-storybook,storybooks/storybook,storybooks/storybook,storybooks/storybook,storybooks/storybook,storybooks/react-storybook,storybooks/react-storybook,storybooks/storybook