feat: add fastify integration section

[multivoltage]

some days ago I opened a pr for some lines about integration with fastify.

Since release 8.0 now fastify supports eta@3

This PR add a section for a basic integration.

[vercel[bot]]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
eta	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 6, 2023 5:48pm

[nebrelbug]

@multivoltage thanks for the PR! This looks fantastic and I'll merge now.