docs: add Fastify-to-Hono migration guide (#21) (#125)

Add docs/migration/from-fastify.md, a side-by-side porting guide for
Fastify developers moving to Nerva/Hono, mirroring the structure of the
Express guide.

Covers Fastify schemas to Zod, plugins/encapsulation to middleware and
sub-apps, decorators to context variables, hooks to the middleware
chain, route registration, serialization/validation (including the loss
of fast-json-stringify response filtering), and TypeBox type providers
to Zod inference. Leads with where Fastify and Hono align (schema-first,
TypeScript-first, return-a-value handlers, in-process testing) and
includes a cheat-sheet table. Linked from the README docs index.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: PAMulligan

README.md

@@ -201,6 +201,7 @@ Nerva is the backend counterpart in a family of Claude Code-integrated developme
 |----------|----------|-------------|
 | Quickstart | [`docs/onboarding/quickstart.md`](docs/onboarding/quickstart.md) | Get your first API running |
 | Migrate from Express | [`docs/migration/from-express.md`](docs/migration/from-express.md) | Express → Hono cheat sheet and porting guide |
+| Migrate from Fastify | [`docs/migration/from-fastify.md`](docs/migration/from-fastify.md) | Fastify → Hono cheat sheet and porting guide |
 | Architecture | [`docs/onboarding/architecture.md`](docs/onboarding/architecture.md) | System architecture overview |
 | Troubleshooting | [`docs/onboarding/troubleshooting.md`](docs/onboarding/troubleshooting.md) | Common issues and solutions |
 | Pipeline guide | [`docs/schema-to-api/README.md`](docs/schema-to-api/README.md) | 10-phase pipeline deep dive |