The Stepper (jig-stepper) guides the user through an ordered sequence of steps,
showing one step's content at a time under a clickable header. Declare each step
with jig-step and project its body via <ng-template #content>. Bind active
for the current index; the app drives navigation (e.g. Back/Next buttons or
header clicks).
Set linear to require earlier steps be completed before later ones can be
reached. The app sets each step's completed.
jig