generated quantities block - naming conventions for posterior predictive checks and cross-validation

[mitzimorris]

Clarify and standardize use of y_tilde, y_rep, and y_sim across the Stan User's Guilde.

• y_tilde is the posterior predictive on new data x_tilde
• y_rep is the posterior predictive given y
• y_sim is simulated data.
• log_lik[n] for log likelihood of data item n

Provide links to downstream analysis tools: ArviZ for Python and bayesplot, posterior, and LOO for R for posterior predictive checks and cross-validation.

Edit: added log_lik for cross-validation.