[addon-design-assets]: Update docs

[iwan-uschka]

Is your feature request related to a problem? Please describe
It's possible to add assets like this

Story.parameters = {
  assets: [
    { name: "Mobile", url: require("./path/to/file1.jpg") },
    { name: "Tablet", url: require("./path/to/file1.jpg") },
    {
      name: "Desktop",
      url: require("./path/to/file1.jpg"),
    },
  ],
};

which is pretty cool. The documentation tells me nothing about the option of giving the asset a name.

[shilman]

we're going to deprecate this addon and are recommending https://github.com/pocka/storybook-addon-designs. closing.