feat: Implement Storybook structure and styling

[lornakelly]

Description

Implement the Storybook structure and styling agreed during the spike ([#88]

Motivation

Make Storybook a more useful development, testing, and demo environment for the editor by introducing a clear folder structure, theming, basic test setup, and removing default boilerplate.

Proposed Implementation

• Colour theme selector (light and dark mode trigger)
• Folder restructuring
• Set up Storybook tests
• Collapsible sections: Introduction, Use Cases, Features
• Remove boilerplate code
• Document features research for potential future implementation

Definition of Done

• Implementation: Fully implemented according to the Serverless Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.