YEF-1106: add answer-first docs LLM index

[Yevanchen]

Summary

• add an answer-first preamble to docs/llms.txt for AI answer engines
• preserve the generated docs page index below the direct answers
• return docs LLM text endpoints with text/markdown; charset=utf-8

Verification

• npm run test:seo
• npm run build

YEF-1106