vehicle-purchase: update introduction

[ErikSchierboom]

Look at the TODO and decide what to do with it

[colinleach]

There are two:

• TODO: Add that fancy concept highlight embed thing to boolean expression
• TODO: Mention ifelse(cond, x, y) somewhere as an infobox

I'm not sure about the markdown highlighting (supported admonitions are note, caution, and advanced), but neither of these seems critical to getting an important, early-stage concept exercise released to a waiting world. It looks fine to me, what do others think?

[SaschaMann]

I'm not sure about the markdown highlighting

I think that was meant to be replaced with a widget which probably didn't exist back then

[ErikSchierboom]

This is the documentation for widgets: https://exercism.org/docs/building/markdown/widgets