Add renv file and instructions

https://docs.posit.co/ide/user/ide/guide/environments/r/renv.html

Start with the linked Posit renv guide and inspect the repository's existing dependency and environment setup. No target files, notebooks, or validation command are named; clarify which R dependencies and instructions are required, then verify that a fresh environment can use the notebooks.

索引モデルが issue の本文から書いたものです。