docs: add a llms.txt file

A `llms.txt` file should be added to the doc, to help AIs to navigate across the documentation and generate Fibbo code. Uses [vitepress-plugin-llms](https://github.com/okineadev/vitepress-plugin-llms) ? Specification : https://llmstxt.org/ Examples : - https://nuxt.com/llms.txt + https://nuxt.com/llms-full.txt - https://llmstxt.org/llms.txt - https://answerdotai.github.io/fasthtml/llms.txt