docs: from busboy to @fastify/busboy

[darkgl0w]

Hello.

Following #300, this PR aims to :

• update the documentation references to @fastify/busboy instead of busboy
• remove the installation step of @types/busboy (@fastify/busboy has its own types definitions)

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[kibertoad]

Thank you!