Add migration guide from Express.js to Nerva/Hono

[PAMulligan]

Description

Create a migration guide helping Express.js developers transition to Nerva/Hono. Express is the most popular Node.js framework — a clear migration path will attract the largest possible audience.

Why

Most backend developers considering Nerva will be coming from Express. A side-by-side comparison showing how familiar Express patterns translate to Hono reduces the learning curve significantly and addresses the #1 question new users will have.

Acceptance Criteria

• Create docs/migration/from-express.md
• Cover these topics with Express → Hono comparisons:
  • App setup and middleware registration
  • Route definition (app.get, app.post, etc.)
  • Request/response handling (req.body → c.req.json(), res.json → c.json)
  • Middleware patterns (next() → Hono middleware)
  • Error handling (error middleware → app.onError)
  • Route grouping (Router → app.route)
  • Static files
  • Environment variables and config
• Include a "cheat sheet" table of Express ↔ Hono equivalents
• Explain what Nerva adds on top of Hono (agents, skills, pipeline, templates)
• Link from the main docs index
• CI passes (link validation)