Migrate the documentation to Storybook and replace the Angular Demo App.

Tasks that are related to storybook but don't block the migration should have the storybook label instead of being assigned to this milestone.

Goals:

• Run the storybook on https://design-system.post.ch
• Document all ng-bootstrap components that don't have a web component replacement
• Place links to the last production deployment of the Angular Demo App on documentation pages that can't display the actual component
• Make the Demo App available in the version switch