Align "home-page info" file priority in build_llm_docs to that of actual home-page generation (pkgdown/index.md > index.md > README.md)

[koefoeden]

EDIT: See comment below. Original message: Currently, build_llm_docs just uses the content in README.md for the high-level overview of the package in LLMs.txt. However, this is inconsistent with the actual homepage, which uses the more sophisicated file prioritization, as seen here: https://pkgdown.r-lib.org/reference/build_home.html#home-page.