Support for llms.txt

[NatElkins]

As LLMs become more important to coding workflows, llms.txt is emerging as a loose standard for putting all doc context in one place. See https://llmstxt.org/ , https://llmstxt.site/ , and https://directory.llmstxt.cloud/ .

I think it could be interesting to support a flag for llms.txt and llms-full.txt. Would make it much easier to add documentation context for F# projects.

[nojaf]

What do you concretely suggest here? Concat everything into that llms.txt file? Nothing that users can't do themselves today I guess...

[nojaf]

I'm having a change of heart on this; I now see the value of having it out of the box. PRs are welcome!