Don't recommend rstan::read_stan_csv

[jgabry]

Submission Checklist

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Summary

It seems like rstan::read_stan_csv() is not guaranteed to successfully handle CmdStan CSVs. We mention read_stan_csv various times in the CmdStanR doc so this PR removes those mentions. At this point I think cmdstanr and posterior provide the necessary functionality that this isn't losing anything important. What do you think @andrjohns?

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Columbia University

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[andrjohns]

Hard agree!