Add vertical scroll bar to "Articles" tab

[TylerSagendorf]

When there are too many vignettes & articles in an R package, the display size may completely hide some of the options in the "Articles" tab drop-down menu. Issue visible here: https://motrpac.github.io/MotrpacRatTraining6moWATData/index.html. Suggest displaying only 10 options at once and including a scroll bar to view the rest.