feat: migrate to nextjs

[americano98]

Summary

Migrates DevHub from the previous Docusaurus setup to a Next.js App Router application.

The PR moves public routes, content rendering, API endpoints, markdown exports, RSS, MCP, sitemap generation, and static assets into the new Next.js structure while preserving the current public site behavior. It also consolidates markdown/MDX rendering, raw markdown generation, AppKit doc example handling, and shared layout/navigation components under the new app architecture.

Key Changes

• Replaced Docusaurus routes/plugins with Next.js App Router pages, route handlers, layouts, and metadata helpers.
• Moved site content into src/content and static assets into public.
• Added Next.js build/static artifact generation for llms.txt, raw docs, markdown routes, RSS, sitemap, and related outputs.
• Reworked docs/templates/solutions/perspectives/hackathon rendering for the new runtime.
• Added Markdown/MDX rendering support for docs features like admonitions, tabs, tables, code blocks, Mermaid, DocExample embeds, and raw markdown copy flows.
• Switched dependency management to pnpm and removed unused direct dependencies left from the migration.
• Kept the product section code in place as a temporarily hidden section; product-related e2e coverage is skipped until that section is re-enabled.

Validation

• check preview
• pnpm install --frozen-lockfile
• pnpm typecheck
• pnpm build
• pnpm exec vitest run
• pnpm exec playwright test
• pnpm exec fallow dead-code
• pnpm verify:images