Addon-docs: Clean up description configuration

[shilman]

Description block should be parameter-configurable for frameworks that don't support auto-extraction. And we have good patterns for that now.

Proposal

export default {
  title: 'CustomDescription'
  parameters: {
    docs: { description: { component: 'some component _markdown_' } },
  }
};

export const WithStoryDescription = Template.bind({});
WithStoryDescription.parameters = {
  docs: { description: { story: 'some story **markdown**' } },
};

[shilman]

Yippee!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.0.0-rc.22 containing PR #11761 that references this issue. Upgrade today to try it out!

You can find this prerelease on the @next NPM tag.

Closing this issue. Please re-open if you think there's still more to do.