Provide a llms-full.txt for the Fastify Docs.

[mcollina]

Prerequisites

• [x] I have written a descriptive issue title
• [x] I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

We should add a llm.txt file to our docs set, so the Fastify docs is more fruible to llms. Here is an example of Anthropic's one: https://docs.anthropic.com/llms-full.txt.

This is useful to teach the LLMs about the new APIs they don't know about.

Motivation

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Example

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