Add migration guide from Fastify to Nerva/Hono

[PAMulligan]

Description

Create a migration guide helping Fastify developers transition to Nerva/Hono. Fastify is the second most popular Node.js framework and shares some philosophical overlap with Hono (schema validation, TypeScript-first).

Why

Fastify developers are a natural audience for Nerva — they already value schema-first development, TypeScript support, and performance. Showing how Fastify patterns map to Hono/Nerva will make the transition feel natural.

Acceptance Criteria

• Create docs/migration/from-fastify.md
• Cover key comparisons:
  • Fastify schemas → Zod schemas
  • Fastify plugins → Hono middleware
  • Fastify decorators → Hono context variables
  • Fastify hooks → Hono middleware chain
  • Route registration patterns
  • Serialization and validation
  • Type providers (Fastify TypeBox → Hono + Zod)
• Highlight where Fastify and Hono align (schema validation, TypeScript)
• Include a "cheat sheet" table
• Link from the main docs index
• CI passes