WIP Fastify tutorial

[jean-michelet]

Reading some feedback from the community that Fastify was lacking beginner resources to learn the framework.
I created the official demo here: https://github.com/fastify/demo

I am now creating an official free tutorial tackling the most common pain points learning Fastify with less opinionated implementation and possible alternatives.
You can make suggestions on this first WIP PR:
fastify/fastify#6239