doc: document R installation paths

[JosiahParry]

For each of the exec envs we should provide the toml that can be used in ricochet-exec-envs.toml

Since the change for exec-envs to be os specific—r-alpine, r-ubuntu, etc.—the r-alpine path is not /opt/R/4.5.1/bin/R like it previously was but instead 4.5.2 it would be nice for us to have these documented

[pat-s]

Given we control these here and only install one R version per minor, we should likely use a symlinked binary without patch version component which links to the respective installed real executable (e.g. /opt/R/4.5/R) and document this practice.