Relative Mean Difference (RMD)
rmd.RdComputes the relative mean signed deviation between `estimate` and `truth`, calculated as the mean of (`estimate - truth`) divided by the mean of `truth`, expressed as a percentage. Positive values indicate overestimation.
This is a thin wrapper around `yardstick::msd()`, with reversed sign and scaled by the mean of the true values. The default `yardstick::msd()` computes `truth - estimate`, while this function computes `estimate - truth`.