adding `ageify` and `vaxify` to vignettes

[papsti]

DE's original comment: "ageify and vaxify need to be discussed. Should this be in a separate vignette for more advanced users, or in the getting_started vignette?"

[papsti]

i think these warrant a brief mention in the basic getting_started vignette, basically just saying they are features and for instructions on how to use them, see the in-depth ageify and vaxify vignette(s). once i've completed the implementation to the degree that we're happy with, i can use my implementation notes to draft the vignette(s) relatively painlessly (i hope).