Add new trial simulation details

Explain somewhere useful/intuitive (maybe in extra info pop-up?) about how the new trial is simulated (e.g. that the average event rate from the meta-analysis is used as the event rate in the control arm in the new study, that a new study effect is taken from the distribution of effect sizes from the meta-analysis).

Start by locating the new-trial simulation screen and the extra-information pop-up suggested in the issue; no file or test is named. Document that the control-arm event rate comes from the meta-analysis average and that the new-study effect is sampled from the meta-analysis effect-size distribution, then verify the explanation is visible in an intuitive location.

Written by the indexing model from the issue text.