[Feature]: Support plugin-llms-txt

### What problem does this feature solve?

https://llmstxt.org/intro.html
https://docs.stripe.com/llms.txt
https://github.com/lynx-family/lynx-website/issues/7
https://x.com/_hisriver/status/1901846870789394589

### What does the proposed API look like?

https://github.com/sveltejs/svelte.dev/pull/979

https://github.com/facebook/docusaurus/issues/10899