Add a Next.js-to-webjs migration guide doc page

[vivek7405]

Problem

webjs's positioning is explicitly Next-adjacent (app/ router, page/layout/error/loading/not-found/route/middleware, metadata API, server actions), so Next users are the primary adoption funnel, but there is no doc translating Next idioms (Server/Client Components, 'use client'/'use server', next/link, next/image, getServerSideProps, route handlers, middleware) into webjs equivalents. A Next dev hits the RSC mental model the AGENTS.md execution-model section warns against with no user-facing doc to correct it.

Design / approach

Add a /docs/migrating-from-nextjs page in the nav with a concept-mapping table, an explicit statement of the no-RSC execution model and the .server.ts boundary, and a before/after code sample. Docs only.

Web-standards fit: Documentation only; it maps Next idioms to webjs's standards-based primitives, adding no code.

Prior art: Next.js Migrating-from guides, Remix Next->Remix guide, SvelteKit Migrating-from-Sapper.

Acceptance criteria

• A new /docs/migrating-from-nextjs page is added to the nav
• A concept-mapping table maps RSC/'use client'/'use server' to isomorphic modules + .server.ts RPC, next/link to , getServerSideProps/generateStaticParams to async page fn, route handlers to route.js, middleware to middleware.js, next/image as not provided
• It explicitly states the no-RSC execution model and the .server.ts boundary
• At least one before/after code sample (a Next page and its webjs equivalent)
• Cross-linked from getting-started and architecture pages
• Tests cover the new behaviour at the applicable layer(s)
• Docs / AGENTS.md / CONVENTIONS.md updated if the public surface changed

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_{Filed from the production-readiness audit (webjs vs Next.js / Remix / Rails / Turbo / Lit). Theme: docs. Priority: P1. Kept to webjs identity: no-build, progressive enhancement, web-components-first, AI-first, batteries-included, close to web standards.}