docs: add Nextra documentation site

[prosdev]

Summary

Adds a Nextra v4-based documentation site at /docs with automatic GitHub Pages deployment.

Features:

• Complete navigation structure (Getting Started, Core Concepts, Guides, API, Advanced, Contributing, Specifications)
• Migrated all core documentation from root .md files to .mdx format
• Vertical architecture diagram showing async processing flow with multiple workers
• Theme-aware Mermaid diagrams (light/dark mode support)
• GitHub Actions workflow for automatic deployment to https://prosdevlab.github.io/eventkit/

Technology Stack:

• Nextra v4.0.0 + Next.js 15.1.6 + React 19
• pnpm for package management
• Static export for GitHub Pages

Content:

• Getting Started guide with quick start
• Complete architecture documentation with diagrams
• Local development guide
• Contributing guidelines and spec process
• API reference and configuration

Supports Medium article launch: https://medium.com/@prosdev/what-happens-after-send-patterns-in-modern-event-collection-systems-0c70e5705a01

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