Add Product Requirements Document (PRD) for Beatstep Marketplace

Commit notes

- Introduced a comprehensive PRD outlining the vision, goals, and functional requirements for the Beatstep marketplace. - Detailed features including a browser-based step sequencer, public catalog, payment processing via Stripe, and moderation processes. - Included user stories, data model extensions, API surface, and metrics for tracking success and engagement. - Established a development plan with phased implementation for MVP and future enhancements.