Generate llms.txt file to make Cotton more AI Friendly

[Benoss]

https://llmstxt.org/ is an emerging standard to help LLMs with documentation.
Cotton is a pretty new library and not very well known by LLMs (yet!) I think helping LLMs can go a long way but this would also enable Documentation MCP servers to use the file and have syntax + examples in the context.

Here is the discussion I opened on the Django Forums that goes a bit more in details https://forum.djangoproject.com/t/generate-llms-txt-file-for-django-docs-ie-make-django-ai-friendly/40672

This would require quite a big refactor of the docs as the content is currently hardcoded in the template. We probably don't want to duplicate the content so this would have to be extracted in JSON files maybe that can then be used as context for the Django templates and as context for a .MD version of each page