docs: brainstorm,brief and prd

docs/prd/index.md

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+# Penguin.AvaloniaUI Product Requirements Document (PRD)
+
+## Table of Contents
+
+- [Penguin.AvaloniaUI Product Requirements Document (PRD)](#table-of-contents)
+  - [Goals and Background Context](#goals-and-background-context)
+  - [Requirements](#requirements)
+  - [User Interface Design Goals](#user-interface-design-goals)
+  - [Technical Assumptions](#technical-assumptions)
+  - [Epic List](#epic-list)
+  - [Epic 1 Details: Foundation & Theme Infrastructure](#epic-1-details-foundation-theme-infrastructure)
+  - [Epic 2 Details: PropertyGrid - Auto-Generated Configuration UI](#epic-2-details-propertygrid-auto-generated-configuration-ui)
+  - [Epic 3 Details: UserGuide - Onboarding & User Assistance](#epic-3-details-userguide-onboarding-user-assistance)
+  - [Checklist Results Report](#checklist-results-report)
+  - [Next Steps](#next-steps)