docs: add Fastify-to-Hono migration guide (#21)

[PAMulligan]

Description

Adds docs/migration/from-fastify.md, a side-by-side migration guide for Fastify developers moving to Nerva/Hono, and links it from the README documentation index. Structure and voice mirror the existing Express-to-Hono guide (#20).

Fastify is the closest Node.js framework to Hono in philosophy — schema-first, TypeScript-first, and return-a-value handlers — so the guide leads with where the two align before mapping the genuine differences.

What's covered

• docs/migration/from-fastify.md created
• Fastify schemas → Zod schemas
• Fastify plugins → Hono middleware (and encapsulation → mountable sub-apps)
• Fastify decorators → Hono context variables (c.set/c.get/c.var)
• Fastify hooks → Hono middleware chain (with a full lifecycle mapping table)
• Route registration patterns
• Serialization and validation (including the loss of fast-json-stringify response filtering)
• Type providers (Fastify TypeBox → Hono + Zod inference)
• "Where Fastify and Hono align" section (schema validation, TypeScript)
• Cheat-sheet table
• Linked from the main docs index (README)

Verification

• All internal links resolve and external links (fastify.dev, hono.dev, zod.dev) were checked to return 200, so the check-links (lychee) CI job should pass.
• Cross-checked against current Fastify docs: hook order, Ajv + fast-json-stringify removeAdditional filtering, and the @fastify/type-provider-typebox API.
• Kept the "Node.js-only" framing precise — only Cloudflare Workers is called out as a target Fastify can't reach (Node/Docker/Lambda/Railway/Fly all run Fastify).

Fixes #21

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