[Feature] Add llms.txt for AI chats

[guillempuche]

Problem
Currently, OpenFGA docs are hard to copy-paste in AI chats. Including the examples https://github.com/openfga/sample-stores.

Solution
You could provide llms.txt format (https://github.com/answerdotai/llms-txt) and add a link to the documentation website.

You could also provide chunks of llms.txt for better easy copy-paste for Cursor or AI chats.