Sensitivity, Specificity metrics formulas

[jxu]

https://yardstick.tidymodels.org/reference/sens.html

For the table, I think you should use TP, FP, FN, TN instead of A, B, C, D to make things clearer like wikipedia does. https://en.wikipedia.org/wiki/Confusion_matrix

Also the formulas don't need to be written in mathjax as they're too wide for the page on my browser and don't use any special symbols.
I think it's much more intuitive to write
P (positive) = TP + FN
PP (predicted positive) = TP + FP
N (negative) = FP + TN
Sensitivity = TP / P
Specificity = TN / N
Precision, PPV = TP / PP

[jxu]

@EmilHvitfeldt I can attempt a PR of this, if you consider it a good idea.